Expansion wise, I'd say wait for a sale on all of them. Get Together, Cats and Dogs, and City Living all come with beautiful worlds. If you like a more Euro sort of vibe, go for Get Together first. If New England/Hamptons is more your style, go for Cats and Dogs. The expansions all go on sale pretty frequently. I wouldn't pay more than half price for any of the ones you don't have. I use Get Together probably the least, but I do find the group mechanic it adds really useful sometimes. If you really like pets, I'd get Cats and Dogs before Get Together. If Pets aren't that important to you, get Get Together. City Living...I love the singing skill and I use the pufferfish death all the time, and the apartments are fun, even if you can't build your own. I've really enjoyed having it, but I don't find it crucial and would probably prioritize it last, unless you really love apartments and karaoke.
Game Packs: If you like family play at all, I'd get Parenthood first. It adds A LOT to family play, and I really wish I'd had it from the very beginning. If you like supernatural life states, Vampires is the best game pack they've ever made, and has the best Sims Vampires and a ton of really cool build/buy stuff. Vampires is just the best. Jungle Adventure and Outdoor Retreat both offer a vacation destination. I think Jungle Adventure is more fun than Outdoor Retreat. Spa Day, I didn't think I'd like as much as I did, but I actually use those objects all the time and I really enjoy the wellness skill and the ability to add additives to your bath is super useful and I really enjoy that.
Stuff Packs: I use Movie Hangout Stuff(I held off buying it because it didn't seem to add much, but I found I really love the items and CAS), Luxury Party Stuff, Vintage Glamour Stuff, Backyard Stuff, and Romantic Garden Stuff all the time. Spooky Stuff is a must for me because it fills a void left by the lack of lifestates. I've enjoyed Laundry Day Stuff, but unless you love laundry...it's non-essential. I bought Patio stuff for the hot tubs and do use them a lot, but other than that, the pack is eh. The rest, eh.
