Let's see:
You could get Ambitions, which comes with the Twinbrook town. The residents are all super-ugly though, so beware if you play longer games and need someone to marry your sim's kids. The EP itself is super-glitchy, so not a good idea.
Pets came with Appaloosa Plains, which was nice, especially for animal lovers.
Showtime came with tiny-yet-cool Starlight Shores, but was absolutely terrible. Don't go there. Worst EP ever.
Supernatural came with Moonlight Falls, which is a nice little town with an old touch to it. Great for people who like magic and historic looking sites and all those things.
And finally, Late Night came with the super-fun city Bridgeport. It'll be very different from the other towns, with its apartments and bars and clubs, perfect for people who want to go down the whole single in the city route, and ties in well with University Life. BUT it's resource heavy and will cause lag because of faulty routing (which can be corrected, but still...)
I say pick either LN, SN, or Pets, depending on what you personally like doing in the game. I didn't mention Island Paradise because Isla Paradiso is still very glitchy just now, and it's going to be expensive because it's new. (It's a great EP, though, with a lot of content, so you could give it a whirl; it'll probably look beautiful with Seasons).