Guess it depends on what you mean by "gameplay", are you looking for things your sims can use or for more more CAS items to better individualize your sims? For more build objects, because building is a big part of gameplay for many players?
For me it's kinda all of that, I would say build items and things for my sims to do are most important for me, though I sometimes can't wait for a pack simply because it adds one particular hairdo ^^
Anyway tried making some ranking on the stuff packs I got:
1. Kids Room as before mentioned, it adds the voidcrittercards and battlestation, I love collections and the kids can uplevel their cards and/or battle their friends/siblings, the puppet theater is so cute, the audience always talk to each other and level up their relationships by doing so and it's just so damn CUTE!
Also adds the best female hair for kids imho.
2.Toddler finally some more things for todds to play with, could finally build a playground + the playdates adds some good gameplay for families (which I mostly play right now), also some sweet CAS and build items.
3. Romantic Garden doesn't really add much gameplay, but I use the flowers so much in my builds, I can't imagine my game without it anymore, so it has to be high up the list.
4. Backyard I love the birdfeeder ;) Also some colorful builditems perfect for some of my builds.
I haven't tried the waterslides yet, but I guess they will add some fun for my sims as well.
5. Vintage Glamour/Cool Kitchen/Perfect Patio/Luxury Party the rest of my stuff packs, I rank them all the same because they have an equal amount of usage in my game, they all add some cool things be it useable items, CAS or build items, but nothing really stands out to me over the others. Wouldn't wanna miss them either!
I tried out the butler once and he annoyed me, I told him to clean but he didn't do it instead he kept cooking meals, which I didn't want or need him to do, since one household member was a chef and did all the cooking (in far better quality), so I fired the butler.