Based on what I personally use the most, I'd recommend either Toddler Stuff or Fitness Stuff. Not saying the other packs aren't worth it (to me every SP was worth $10 except for Luxury Party and MFP) but I use those two the most.
I'm a legacy player so I use Toddler Stuff constantly. CAS and build/buy, mostly. The toddler gym items are a bit large.
I actually love the build/buy items in Fitness Stuff for public gyms. Plus I always use the CAS for my athletic outfits. I occasionally use the workout videos for my Sims if they need to exercise but I want to keep them on my lot, and I use the headphones for my music-loving Sims whenever they are doing other activities.