I'm disappointed that all Sims 3 stuff is the same price
I need to choose one EP to buy for it right now
All of them.
But...alright, so lemme give you a quick rundown- in reverse order of release:
Into the Future - Unexpectedly the best one, which was really odd considering the expansions that segment content off to some other world are usually YMMV, but I was legitimately blown away with all the ideas they had for this one. The "choices in the past can affect the future" idea was especially cool. It's the last TS3 expac and honestly the one I'd...not recommend the most but definitely heavily recommend. It's hard to give it the huge thumbs up when it doesn't add as much to the main game as some others do.
Island Paradise and University Life - Uh...alright, so after...either Supernatural or Seasons I'd taken a break from the Sims, and when I came back it was after ItF came out. I had about 3-4 exps to taste but weirdly I don't remember using these much, which is a shame because they were supposedly pretty good packs. The traits were a nice addition and I did use those as well as some of the added careers but overall I just didn't live by the sea usually and didn't have any college-bound sims in the Legacy I played, which was a bit of a shame because I spent a lot of time in College back in The Sims 2 University. But yeah, if you don't generally live by the sea and don't want to, Island Paradise doesn't really add much to keep the average player invested (though the resort system, which I've only heard about, would probably have eaten most of my time if I'd tried it out since I looooooooooooooooooved The Sims 2: Open for Business) and if you're not interested in sending sims to college it doesn't either, though if I had to recommend one I'd probably say the Island if only because it can be utilized whenever and doesn't really require commitment. But again, both are solid packs.
Seasons - I was never as big a fan of the Seasons packs in the Sims as some people but it's the one that the majority of the fanbase looks forward to with bright eyes. It made sense with this one since it even added Holidays, and even more, Seasons is kind of one of those expansions that affects the game fundamentally in a way that makes it almost essential. It adds a lot of interactions and things to do, and while I wouldn't personally put it in my top 3, it's definitely up there as one of the best and certainly among the most essential.
Supernatural - And then there's the dark timeline. Look, Supernatural...exists. It's not remembered well, mainly because it wasn't really necessary, and it's...just...I mean. It adds a bunch of different sub-types for characters (Vampire, Witch, Werewolf, Zombie, Fairy) and each subtype has different characteristics and interactions. If you've been really pining for that, this is for you, other than that though if you're going for all of them I would recommend just waiting until you're collecting your last few expacs to get this one. It adds flavor to the game but not one that's really all that substantial.
Showtime - The Get Famous/Superstar of this game, though the fame mechanic was added in Late Night so...like those but without the fame mechanic. This one honestly wasn't that good and even remembering it, I don't have much to say about it. It adds some new careers, allows you to play at venues...if you don't have Late Night it really is even more lackluster. Honestly looking back I'd probably say this one was worse than Supernatural because at least the sub-types were fairly interesting in that, I struggled to find things to do with Showtime. Nice to have if you do but I wouldn't recommend it.
Pets - The best Pets expansion The Sims has ever produced and the most essential expac bar none.
None. I'm not even that big of a pet guy but good god do they do some fun things with this game. While in the past it would add only a few pets, with The Sims 2 adding simply cats and dogs and that was just kind of the expectation, The Sims 3 decided to take advantage of its open world and add wild creatures. Granted, you can only own and control cats, dogs, and...horses, interestingly, but in the world you could also find deer, raccoons, lizards. Some you can collect and put in cages and name (my legacy family had a bird and a pet turtle, the former of which are cat really clung onto). The customization of the pets is fun and a lot better than it was in the past, walking around your existing neighborhood and just trying to find all these different animals or befriending and adopting dogs and cats and stuff like that, like...I dunno, that added more flavor to the world than any other expac in my eyes. There was more added, of course, but I'll just say that it's definitely worth it if you don't already have it, it's just a lot of fun, really makes the world feel alive.
Generations - ...Y'know, this one should've been good for me but I actually wasn't much of a fan. Granted, the addition of memories was cool and actually being able to have birthday and wedding parties like back in the other games was...
...Alright, okay, I say consider this one. Its whole purpose was to give life stages more to do and more interactions in between each other and while personally I think it met that goal with mixed success, in the long run it does add some pretty important stuff. Some of it is kind of intrusive though.
Aaaaand I've gotta go. I'll write up the last three when I get back since I have the most to say about those and there are stipulations to go with them, but for now I hope this helps!