For what it's worth, this is the order of what I'd buy first to make a decently rounded selection of worlds and gameplay. Some newer packs might not be here because I haven't bought or played them a lot, but I do own most the big ones. Also, I'm a big user of create a sim custom content, and build-buy custom content so although I like getting new CAS and BB content from a pack I am often a bit more excited by worlds and gameplay additions.
Get Together is my first choice. For a start it has a really good world - it's TS4's biggest world, and it's well designed with a large number of lots in different environments and just generally it's a pretty world too. And for gameplay it has things like clubs so you can actually create events where sims take part in defined activities, which otherwise just doesn't happen, they're all off doing whatever. It just gives you enough space to build and make a decent amount of locations to visit, and houses for sims to live in.
Alongside Get Together I think Castle Estate is really worth it. The items are well picked. You can make a lot with it, like a castle (not to be too obvious!), also churches, ruins, a city wall/gate. It goes perfectly with the world in Get Together too as it's the kind of place you'd find these buildings. Being build items I think you get more for your money from it than other kits that contain items to decorate a room or two. I've made multiple distinct buildings from it.
Next, Seasons. It does so much to make the worlds keep changing day to day, week to week. Really makes it feel more alive. And it creates seasonal activities and holidays, which I just enjoy. Also, it works really well with the Get Together world giving it a full range of seasonal weather and events. I like the addition of seasonal outfits in CAS and getting to style sims with different outfits for the season.
After that, Island Living or Jungle Adventure. They have similarities and differences. Both worlds are beautiful and have unique environments and cultures that are distinct to the base game worlds and Get Together's world. Both are geat to explore too. However Jungle Adventure being a game pack versus expansion pack like Island Living is smaller in every way. However, I still think it does a lot. I love the archaeology gameplay and the temple exploring. I also love the world - but it is of course smaller so you won't get to do as much building, and Jungle Adventure is the first pack I've mentioned where your sims can't live in the world, just visit. There is a mod to change that, but if for any reason you don't want to use the mod then this difference is important. If you're happy to use the mod or don't mind it being visit only then I would significantly raise the value of this pack as an alternative to Island living.
At this point you'd have a wide range of world types and some different cultures, and lots of good build-buy items and clothing. From here I think it gets harder to know what someone else would like next. I mean, like do you want your sims to go to university, or do you want them to be werewolves?
I'm going to give a couple of notable shoutouts though for a few small reasons, not to say they don't have other great stuff, but these bits are game changers throughout the worlds:
For Rent lets you build multiple homes on one lot, either distinct buildings or apartments, it's very flexible. This really changes all the worlds by making the capacity for sims go up a lot.
Buisnesses and Hobbies is able to make lots both home and buisness lots, which makes for some space savings and new ways to play.
So if you're running out of room for builds and sims, these are great next packs. Plus they both have great worlds and other gameplay.
