Well this may sound strange but I have two different combinations of expansions installed on two different computers.
On my mac I've got the base game + World Adventures + Ambitions (the most played combination)
On my pc I've got the base game + Ambitions + Late Night (the combination I play just occasionally)
Why this distinction? Well, when I bought World Adventures and Ambitions they seemed to fit very well with the base game, adding new depth and enhancing the gameplay, but at the same time leaving its structure unchanged. I mean, I hated vacation themed expansions in the previous Sims games, but World Adventures was such a big improvement overall (and so was Ambitions) that I instantly loved it.
Now, I like Late Night so much, but it just seems not to fit the game as well as the two previous expansions. I mean, celebrity system and vampires are cool, but I don't like the idea to have them mixed with my game when I don't want to. In my eyes Late Night is more like a standalone episode than an expansion I actually want to see added to families I usually play with... it brings no significant improvements to the base game, and the new professions, the traits, the lifetime rewards, they seem to be made for Bridgeport and Bridgeport only. So when I play Late Night, I play it only to explore the features the game has to offer (I've never started a game in a town other than Bridgeport, and every single file I've started, it always featured actors, directors, mixologists, vampires, bands etc. in a way or another), but when I want to simply "play the Sims", I go back to my "regular" Vanilla+WA+Ambitions combination.
Oh, on a side note, I don't like Pets and Showtime at all, though I'm really looking forward to Supernatural... I'll definitely buy it!