If you were to buy just one expansion...

[amyrose]

Ambitions. I played with just base game and ambitions for about 4 months, and I never got bored.

[Axiom]

Oh dear, so hard to make up my mind...
But I'd have to say:
1. Generations
2. Ambitions

Generations comes with all sorts of neat family related stuff [Even shelves, thank you EA!], and I love the bunk beds for when my Sims family is full of kids.

[Superbad55]

I have the console so I only get Pets (and that even is not an expansion pack!)

But if I have PC it would be a tie between:
- Generations (spiral staircase, bunk [I think that Generations has bunks])
- Late Night (Mixology, the muscle definition slider)

[kindofagirl]

Ohhh it's hard to choose one, I think I would choose generations

[MoMoll]

This question comes up a lot. What excites you? Do you like seeing if you can master new careers? Do you like travelling and exploration? Do you trying to manage families with their rebellious children would be interesting? Do you like to party and hob nob with celebrities? Do you want to to raise animals and race horses? Do you want to be a star singer or acrobat?

You see, it depends on what you like and your personality. Personally, I love Late night and Showtime, because I love the dance clubs and singing. I also like pets, because I love horses and cats.

[Noshmek]

Ambitions, because I cannot imagine simlife without a time machine.

[Stiborski]

I can see Late Night is kind of the underdog, huh? But it'd probably be my pick, since I'm a party monster myself (I got out last night and came back home 7am, haha) and I love big cities. I hate everything related to celebrities, though. It's really annoying.

[Korva]

#1: Pets. I love animals, they just add so much enjoyment to the game (and RL). Also, Appaloosa Plains is a lovely town for the most part, IMO, the best of the "official" ones.

#2: Ambitions. Self-employment and the consignment store are a must for me, since I pretty much hate rabbit hole careers due to the fact that they take control away from me for the biggest part of the day; add sleeping, hygiene, travel times and other basic chores and there's barely any time to actually play such a Sim for fun.

[Mikeyspoons]

I think mine would be a tie between Late Night and Ambitions. Late Night because Bridgeport it my favourite neighbourhood after Sunset Valley and I like the people in it. I also love the piano, no mansion is complete until it has a grand piano. But Ambitions has the firefighter career which is probably my favourite to play and I enjoy the stylist too as I can amend the clothes on all the town members and a fair few need it  So I don't think I would be able to decide between the two.

[xFezIsAFreakx]

I don't think it makes sense to make this decision until all the EPs come out, but for now - Ambitions (Gen is a super-close second) and for an SP - I'd take HELS.

[Coocoo girl]

I would get showtime, because, other than the stuff packs, I'm pretty sure that's the only one I don't have!   But I don't have any of the stuff packs.  I want to enrich life for my sims. lol