Sims 3 is having a Steam sale! I want to get it on my Steam library but does anyone have recommendations for DLC? There are so many!

If you want to actually expand gameplay, get any of the expansion packs. Avoid the "stuff packs", they just add random furniture and clothing you could just get at mod sites. I would recommend "World Adventures" as one essential expansion pack, but any of the others are fine.

@FrankCanada97: Can you explain why? I remember playing it a long time ago and I thought it was kind of a weird distraction instead of something that adds to the sims stuff.

Pets and Seasons add the most vanilla and expansive content. Generations is easy to skip over. Showtime, World Adventures, Ambitions and Late Night all add some nice extras and depending on your taste, Supernatural is either really nice, or practically gamebreaking as everyone in your neighborhood will end up with wings, wands or fur. Like @FrankCanada97 said, the stuff packs can be ignored.

Out of all of them, I'd say pets is the best expansion.

@audioBusting: Why I recommended World Adventures? I guess it was provided a compelling variation to the traditional "Sims" gameplay experience. Like I said, any of the other expansion packs are just as good. Why I say stay away from stuff packs? I don't feel it gives enough bang for buck in what it's offering. The Sims has a pretty big modding community so you could get some new furniture or clothing items from those sites.

Sweet, I can buy more Sims for my wife- wait a minute, I already bought her every single one so far. DAMN YOU EA!!! 
 
If there is just one you should buy, definitely get Pets. It livens things up a lot just by adding animals. Something always felt missing to me when a new generation of Sims games would come out and Pets weren't included yet

Late Night is basically Superstar and Hot Date from Sims 1, so that's a neat addition.

But, in general, avoid the Stuff nonsense.

Supernatural and World Adventures

Thanks everyone, I think I'm just going to get Pets for now and get other ones later.