Since my opinion might be quite different than yours would be if our positions were reversed, I'll just list why I like the expansions and you can judge for yourself if it helps or not.
WA: Much more than vacationing, I adore the "buydebug" items for building. I like the wall plaques that you can set your own text, the floor switches and the hidden door in particular, but it requires a lot of familiarizing with how they work (the most helpful resources for that can be found here). I have also occasionally used traps for purposes other than they were intended, but this is less appealing without some of the other expansions. Also, you can buy chests in the stores in other countries that are incredibly useful for storing things, and I use them a lot. Plus it adds my favorite skill: Photography. It's so much better for recording generations past than painting is.
Ambitions: One of my favorites, this adds several extremely amusing professions as well as the ability to become employed through a money making skill. Plus it adds the Inventing and Sculpting skills, which are my second and third favorite skills in the game respectively.
Late Night: The best things about this game are also the worst things: Celebrities and Vampires. Both are really cool and fun to play, but both are impossible to avoid if you're not looking to deal with them. It does add 3 new musical instruments to the game (and corresponding skills), but really there's not a lot you can really do with this aside from playing for tips in the subway in Bridgeport and forming a band (which may or may not be bugged at the moment). The new types of hang-outs seem really neat in theory, but in practice they're just so-so. It also includes one of my least favorite skills: Mixology.
Generations: I don't have this one yet, but I will sometime this week.
Pets: While this is, without a doubt, the best pet related expansion for any of the three versions of the Sims, if you're not into animals that much you'd be better off getting something else. It has some of my favorite outfits in the game, and horses are extremely cool...but they require a lot more effort than a working Sim can feasibly put forth before collapsing from exhaustion.
If I had to recommend one out of the whole lot, right now it would be Ambitions.