I think it's really up to your own tastes. I don't own but a few.
I have...
Country Kitchen. I like rustic/country decor and at the time this came out we really didn't have many options in that style. Granted Cottage Living came out after but it compliments that style very well and I use the items quite often.
Throwback Fit. I bought this for my decades' challenge, but I actually use the items more than I thought I would. As said above there isn't much in children's clothing without resorting to CC so this brings a few items for them. I like this CAS kit. It is right now the only CAS kit I have.
Little Campers. I am a generational/family gameplay type player and this kit is a must for the kiddies. I also like to build, but it takes me such a long time when building these days so I don't do it as often as I use to. It has some unique DIY pieces that just fit that gameplay style. My kiddos use the items a lot.
Courtyard Oasis. I just recently got this kit, but only because I broke down and bought the Wedding Gamepack. It compliments that pack along with Oasis Springs. It is a specific themed pack so not sure if there is a lot of use for it outside the Mediterranean style and vibe.
And that's it. I am thinking about getting the Icheon kit but still on the fence about it. I do like the style of clothing and think it has some great pieces. I also thought about getting the Kids Fit Kit for more clothing for child sims since there isn't much, but again not sure. I don't mind using CC so for now I'll just hold off. I may in the future get the clutter kit because I love clutter in my builds. On the fence with the pastel kit, I think it is cute but I'm worried about how often I'll actually use those items, I do like having more wallpaper options for my game.
Outside of that, I can't say I'll ever get the other ones we have currently, though I do like the wallpaper in Decor to the Max, I just don't want to buy a kit for one item. Now if these kits had some premium content offered in them as the Sims 3 store collections had, I'd probably buy them all.