To me, either Bridgeport (Late Night's city) or Riverview (the first downloadable neighborhood and, on my especulation, the intended second neighborhood to ship with the game). Though I have trouble with both of them.
From the base game to Late Night, EA made progressively integrated nieghborhoods with more and more complex stories, and the effort they took with Bridgeport shows: nearly every household is related to the other ones and almost all of them have interesting settings and conflict. You have a kinda main story with the clans of vampires, and around them several quirky characters. My problems with that, however, is that they didn't go far enough: I expected vampires brawling on the street, maybe a Van Helsing/Buffy-like vampire slayer, a more involved storyline... but as with every TS3 world, they had too many sims (understandable, since the city wouldn't function otherwise), and not all of them were really interesting. The story wasn't really very developed, and you didn't get a really good grasp of who the characters (sims) were. That was a problem woth every TS3 world, though.
Which each expansion, EA tended to include too many young adults sims, so you didn't have a lot of variety in characters and the town ended up empty after a couple generations; Bridgeport was a especially egregious case in that respect, which seems like a missed oportunity to me, too. Would have loved to have teenage bands and brawls. Finally, the city was designed as a real one, which translated in really empty streets during the day; that broke quite a bit of immersion.
This is not a roast of Bridgeport, though; the city is beautifully designed, it has interesting ideas, and every building has some hints of a story, which I really appreciated, giving it a 'real' quality to it. EA also took the opportunity to make some satire on real cities and celebrity culture, and even some real-life celebrities; I don't consider it as brilliant as the satire in TS2 and specially TS1 (from TS3 onwards I consider them going very 'soft' with the developer's decisions), but it has some really good and funny ideas thrown into it.
Now, Riverview... that was really well-done world. Quirky characters with interesting stories, though with not a very developed storyline (having too many relationships on TS3 is really a hog and difficult to mantain, so I understand them not really programming a lot of relationships into the game) and a really great design that took great advantage of the game engine and scope. You really get a feeling of a town out of Riverview, and dividing the city into very differentiated districts was a tremendous idea. Plus, the lots are beautifully designed and well-placed, and you get different socio-economic backgrounds and conflicts in each family, and it executes it's mid-century americana-like rural town theme really really well. Also, it kinda has some TS1-like quality to it, helped by the fact it includes the teenage Bob and Betty newbie. Man, I love that town.
I'm also really partial to Moonlight Falls, which I believe did that 'brawling supernatural clans' theme better than Late Night, and has a really interesting backstory; but I don't own Supernatural, so I can't really judge.
I really like the idea of several downloadable neighborhoods, like Roaring Heights, Monte Vista, or (the better one in my opinion), Lucky Palms. They didn't really go too far with the story of those ones and they ended up just being a collection of shot-outs to more entertaining shows and movies. The most pitiful of those is, however, and on my opinion, Barnacle Bay. They had an opportunity to make a Maine/Nantucket-like pirate town, or even a Caribbean one, but it ended up being an enormous bluff full of single young adults. When will EA learn playing one sim is normally the most boring way to play the game?
Quite a divagation here, but those are my two-cents. I'm aware I tend to focus too much on character and story on the game, so maybe I'm not the most partial player.
Monte Vista has the most detail put into its lots, but I find the Sims in Monte Vista a bit boring. Sure there is a shout out to Goopy Gilscarbo and his family, and I think the Capp and Monty's too, but therein lies the problem, Maxis/EA are not good with storytelling. They just kind of shoe-horned these families into these worlds for the sake of nostalgia rather than to further tell their stories.
Because the earth is standing still, and the truth becomes a lie
A choice profound is bittersweet, no one hears Cassandra Goth cry