The best EP by far is Seasons.
Get Together has my favourite world, Windenburg, which has a really European vibe to it, but it's not that useful otherwise. It has disco dancing too, but to be honest, the dancing isn't much better than the base-game stuff.
Gamepacks are totally worth it, and often times better than EPs.
If you like dark-themed stuff and don't mind supernaturals too much, Vampires is absolutely the best choice. It's got some of the best spooky 'stuff' in the game, and vampires are done pretty darn well too. They're not very obnoxious if you choose to ignore them.
Jungle adventure also has some really great spooky looking stuff, and lovely scenery.
Parenthood is great for making family life more... eventful. So if you're intending on playing families a lot, I'd recommend that one too.
Stuff packs... aren't really so worth it.
I'd probably only recommend vintage Glamour, it has some objects with game-play like the butler, the globe, the dressing table. And really nice clothing as well.
Spooky stuff I regret buying. The decorations are way too obvious and childish, and the clothes are basically all costumes like witch and zombie. Not really anything that's usable unless you're at a Halloween party. Vampire stuff is much better if you're looking for that sort of thing.
Onto custom content - I can't tell you if there are any major creators, because I usually just fish all over and grab what I can.
But there are three main types of CC that people subscribe to: Maxis Match (Fits the look of the game), Alpha (More realistic CC), and Clayfied (Alpha hair that's been turned maxis match, it looks like a mix of the two.)
The sims resource is great for Alpha. Maxis match and clayfied you might have to do some of your own searching for through google and pinterest.
There 'are' Easter-eggs and little things that add story-line... but they're few and far between compared with previous iterations.
