My favorite is Unleashed because I love love love the music and New Orleans references. I love the additions to the neighborhoods, the extra families with their back stories, the gardening and the pets. It was such a slice of heaven to get that expansion. Makin' Magic was a close second, but, of course, all the expansions added so much to the game. Possibly my least favorite was House Party only because I missed having groups of Sims over without it turning into an event, other than that I enjoyed the parties and the objects.
The thing is, it's easy to forget that the base game of The Sims without Living Large does not quite seem like the Sims. So, while Living Large is not what I'd ever consider one of my favorite expansions, it is a vital one and helps to define the very character of the game (roaches, genies, the concoction station, the telescope, the love bed, servo, the tragic clown, etc...).