From the top of my head;

UNI: I love taking my sims to college, but I eventually got a shorter college mod
AL: apartments are a must in every neighborhood I create, I can't build lots to save my life but everyone else makes beautiful apartments
SEA: it remains my favorite expansion pack of all time simply for the seasons. I have a 30-day season mod because less-than-a-weeklong seasons don't cut it for me

Well, I only have the first three EPs (University, Nightlife, OFB) so obviously I don't use any features from the later ones. I have added OFB and University to my original game, but, when I think about it, I mostly use the features from Nightlife and base game, which I've had from day one. (I started with Double Deluxe.)

Probably the feature I use most is un-scored outings, typically formed on the fly by (if necessary) inviting friends round, and then using the Ask...To form Casual Group...Just for Fun interaction. Once I learned how easy to play dates are, between Sims who love each other, I also play a lot of dates, and the pleasure of seeing two Sims ecstatically happy in each other's company never palls for me. My Sims tend to have very full love lives (possibly because I have none myself).

I have though studiously ignored the whole "chemistry" system, giving all my CAS Sims the same turn-ons and turn-offs as myself. (It does help me to empathise with them!) Actually I gave Andrew (my very first CAS Sim) Facial Hair as a turn-off (he's gay but he doesn't like beards -- they get in the way when kissing!). But every single Sim I've made since (starting with his mum Gloria) has Vampirism as a turn-off, reflecting my dislike of supernaturals and the occult. As for turn-ons (though I have mentioned them before) I'm embarrassed to say, but I think you can guess from the clothes my Sims wear! They tend to sleep in their underwear, not bother getting dressed when they get up, and they spend a lot of time in the hot tub (usually a Love Tub).

I think I am one of your sims, except I don't have a hot-tub, so I spend all day in a hot bath instead!

Ho god, I just realized I barely use any of the EP features... I used to be really into apartments, but kinda stopped because the buildings "looked bad" compared to ones from MTS... which is a strange reason because I barely pay attention to what buildings look like on the outside, and on the inside they were pretty nice. I also barely use uni these days... ironic.

Maybe the downtown from nightlife, and the chemistry systems, those are pretty fun. And the aspiration benefits, especially the romance chemistry boost, that's vital with a mod to make falling in love and crush harder with low chemistry. I also like to include driveways and at least the cheapest car these days, I don't know why. Also, changing the seasons and gardening, I kinda started doing those more often. I should also similarly have uni students and apartments, now that I think about it.

And does simply causing Drama (tm) count as feature?

Nightlife and Freetime are the ones I consider "essential" for my gameplay. I probably would have included Seasons too if I had it installed, but I'm assuming it took a toll on my computer or something because I've had it uninstalled for years and there must have been a reason for it because I really like it. Besides the weather, I miss fishing and the hanging out interaction from it.
I think both NL and FT had such all-encompassing features that it's hard for me to imagine my game without it. Many of my sims live in or visit Downtown, and many have the pleasure aspiration. Chemistry usually play a big part in my sims' love- and sex-life. Lately I've been playing more with groups and outings, aside from dates.
Freetime has my favourite career tracks. I also really enjoy playing with various hobbies for my sims and the items that come with it. I'll also regularly check in on and give out the aspiration perks and secondary aspirations. The knowledge "summon aliens" is the one I like best and will only actively attempt making sims alien-pregnant through that rather than trying to have them abducted via stargazing. The ability to walk to community lots is also one I utilize a lot, especially since not many of my sims have cars.

I send my sims to University, if they have the want; when I lock the want depends on their interest in school. So, I use University a decent amount.

I use Nightlife a lot. I love sending my sims Downtown, the dates are a cute idea, outings can be a blast, Grand Vampires are delightfully goofy, and the Chemistry system is just brilliant. It gives you an idea of how attractive sims find each other. Nightlife is my most utilised expansion!

Open for Business... I forget I have it installed. I've thought about starting some businesses up, though, because I've done it before and I remember having fun.

Pets... I'm not a massive fan of pets, personally. I get them for my sims sometimes if they want, but for the most part, they just exist. And the small animals I wind up getting for them... forget about them, too.

Seasons, I love it! The changing of the seasons and having the ability to put away leftovers is amazing. I can't imagine a game where I can't put away leftovers!

Bon Boyage, I just like it because of the jewelery option. Going on holiday is neat and the secret lots add that extra replay value, but it's not something I do often.

Freetime is really good, in my opinion. I love the hobbies system, talent badges, secondary aspirations, and aspiration benefits. I especially love the secondary aspirations, because it adds that extra bit of dimension to a sim's personality, as do the hobbies.

Apartment Life is my most underutilised! Witches are cool, but that's all I got from this EP. I've began to use apartments for fresh graduates who don't have enough money to buy a house and their parents aren't willing to help, but other than that... I don't use it all that much.

In short, I mostly use University, Nightlife, Seasons, and Freetime.

I don't like playing apartments. Perhaps, if the vanilla game had some of the options mods offered today, like shopping enabled, etc. I may have - but I can say that I did play all the apartments in the bin once and that was enough for me. Yet I do use the build/buy stuff - a lot!
I actually think I use all the other features:
Lots of owned businesses, which I have enjoyed playing even in the vanilla game;
FreeTime going very well with owned businesses and the pottery shop is doing great in my hood;
Seasons is lovely and I have some farms going too,
and, as everyone knows by now, I love Uni.
Some of my sims have pets too and there is a sim who owns a pet shop too.
And I still like the basegame townies

In no particular order, I use the most out of these packs:

Apartment Life: I'm not a fan of the apartments because they're glitchy, so I mainly use the witches that come with this pack. My favorite occult in Sims 2.
University: If I never played Wants based, I'd send all my teens to Uni every single time - out of character or not! I really enjoy the gameplay. I also like the new careers, clothing & objects it came with so I use the features quite a lot.
Open for Business: I use this EP a ton! I love running businesses but I'm not so fond of running restaurants. Also... Give me an army of Servos!
Freetime: I use it quite often. I enjoy the talent badges, hobbies and the activities.
Nightlife: I send my Sims downtown pretty often and I like how the cars worked.

I do use features from Pets & Seasons but they're just sort of there so I don't count them as features I use often. As for Bon Voyage, I do send my Sims on vacation but only once in a blue moon. :D

A few weeks ago, at the back of a Dobbies' Garden Centre, I actually saw a Real Life hot tub for sale. Which made me think how familiar these things are to my Sims, whereas I had never before even seen a real one myself! The price tag of the real one was comparable with the §6,500 my Sims have to pay. It's well beyond what I can afford, and anyway -- a problem I share with a few of my Sims -- there really isn't room in our back garden. Now not many of my Sims can afford §6,500 either, but most of them can lay their hands on the 20,000 Aspiration points needed for a Love Tub. So I wonder if I've got any unused Aspiration Points in my Inventory . . .

. . . No. Even if I could get a hot tub or a love tub and put it in my back garden, I wouldn't be able to do what I really want with it -- soak and chat with some of my lovely Sims . . . and maybe even get a cuddle from one of them. I'll have to revert to my original plan of somehow becoming a Sim and moving to Veronaville. There's plenty of room for a hot tub in the back garden of 30 Poet Place, the little house that I want for myself in Veronaville.

Believe me, AndrewGloria, they are maintenance hogs with limited lifespans. We have a dead one, my parents' second, they've owned the place for thirty-five years now. It's been dead for over a decade. It costs more to get rid of it than anyone wants to pay, and then we're left with a hole in the deck that needs to either be rebuilt to get rid of the hole or instal a new hot tub in the same place. The deck needs rebuilding anyway . . . someday we'll get to it. I'd rather have a sauna.

They are, however a fairly standard feature at mid-range-and-up hotels in the US, so if you ever happen to travel overseas, take advantage of someone else doing the maintenance and enjoy a soak!

I saw a picture that someone posted on social media about an outside bathtub they installed, which sounds like absolute heaven. They built a trellis and pergola around it so that the plants gave them privacy, and they had an outdoor electric water heater to fill it. Nice drink, good book- I could go for that, watch the sunset and listen to the birds.
I often give my sims outdoor bathtubs, and one character from Sims 3 (Lucky Perkins) has an outside bath too! After all, who wants to sit in a tub of water with other people (except for Sim Sample)? I like the bathtub because I can get away from people!
(I do love hot tubs for my sims though- maintenance free, I think they were originally supposed to be breakable but they took that out of the game before release- and very social. Make friends fast, and gain comfort and fun! But, they get cold in winter even though the tub is heated. That's when the saunas come in handy!)

I was doing a little photoshoot for a household in my BACC, when Ricardo did this while looking right at me. So unnerving.

Never had anybody rich enough to move into 210 Wright Way, so I never played that house (https://sims.fandom.com/wiki/210_Wright_Way). That place is huge! I moved in Alexander Goth's family and even with three kids and a dedicated game room, I have no need for the entire third floor. What do I do with that?

There's got to be some way I can make use of that empty attic that would be fun storytelling. Maybe he'll stash Lucy up there and get married again.

Oh! That's the lot that if Don goes through with the wedding to Cassandra the newly wed Lotharios move into.

Any Family sim worth her salt can tell you exactly what to do with all those empty rooms. Have ten babies, that's what!

I remember using the familyfunds cheat to move into that house when I got Seasons as a kid, it was my favorite house to play; except for the times lightning would strike the random shrubbery in the backyard and I would call the fire department... only for the fire to put itself out by the time the NPC got to it .

Apartments, Seasons and University. All my hoods have apartments and a custom boarding house for the very poor (rooms start at $60 a week, and with bunks two Sims can share that space!); a lot of my Sims grow their own food; and so many go to university that I got Cyjon's shorter semesters, as it can get boring otherwise. Most of my hoods have subhoods, but other than that I don't make much use of the Nightlife and OFB features.

If I could only have one EP, it would be Seasons, and poor Sims would have to start out mortgaged to the hilt.

I've got a rudimentary map worked out for the 6 different SC4 terrains I'll need for my medieval world... now if only I could come up with NAMES for more than like two of the kingdoms there! Not being able to refer to places by name makes it way harder to come up with ideas for them!

Zarathustra doing a medieval hood? Color me intrigued. You want names? I got names for days. If there was such a job I would be a professional namer-of-things. Here's some random mashup freebies: Jarskgrog, Wintouria, Clavishdon, Plarkdom. Here's some simlish-themed freebies: Aspiratia, Platinumion, Llamascher, Plumbobbshire

Here's a site that might help you out: http://chaoticshiny.com/index.php

It has a map generator, name generators, all kinds of fun stuff to put together a medieval land and city.

Haha, thanks- at this point I've already done too much of the "two English words mushed together" approach to naming things (Westfall, Iron Circle, Stormwall, Highwinter, Crossroads Hills, etc.), so I'm trying to come up with names that are more original and that I can turn into adjectives and alter (as happens with real world language) depending on where in the world something is being talked about...

This project is really one that I'm doing more for Sims right now, but I'd also like to flesh out enough that I could use it for a tabletop RPG setting or for writing stories unrelated to a Sims build someday too, so I'm trying to figure out as much of it as I can, and at the moment only being able to think of places as "the deserts way south of Westfall where vampires are from" for that society, or "the massive empire east of the haunted forest that's totally not just stealing the aesthetic from Imperial China (wink wink)" among many other locations I still need to name doesn't really make it easy to come up with more detail yet!

What I REALLY need is a name for the world as a whole! I can't even group things together properly without that!

Listen to sims talk and find a word you think would be suitable. Like how Marticore named a planet Elaganda after what male sims say when there's a plate in the way

Playful sims are right - jumping on the couch is fun. Then again pleasure would definitely be my secondary aspiration. Not even gonna try the flips and stunts they pull, though.

Jessica Peterson and Marissa Cleveland are now my favourite pairing.

Anyone else too obsessive compulsive to actually enjoy their game? Feels like no matter how much I tweak and/or mod my game there's always ten more things that bother me still, to where I spend my 'playing time' actually just googling for mods and opening SimPE. It's starting to bother me now. No wonder my hoods barely get off the ground.

@Bubblebeam I have realized that I played more seriously when I used worse computers, perhaps that everything was more restrictive made me focus on the real game without distractions.

I would also like to test the game on a laptop, once I used it on a friend's laptop and it felt like a kid.