Must Have Kits??

What kits you like greatly depends on your personal interests. I like looking at sims community or YouTube reviews to see all items included. 

My personal favourites 

• book Nook kit: customise bookcases with modular book cases and nice and cozy couches. I use this kit in practically every build .
• Little campers kit: nice kit with several usable items like a doll house, swing set, games table, . . . Great for family homes 
• Every day clutter kit: nice accents throughout your building. Especially useful if you like to build 
• Cozy kitsch kit: I didn't expect to use it as much as I do. The couches, chairs and lights are very useful in almost every building 
• Modern lux kit: very stylish bedroom with a vanity table. A cohesive set for an entire bedroom. The cushions are great to make all couches look more cozy
• Story book nursery: I usually go for the lighter colour swatches but very much like it for my youngest sims

Honorable mention: castle estate kit: great and very complete set to build (but not furnish) a castle. But it's very niche. 

I use the kits I mentioned here in the majority of my buildings and am very happy with them. CAS kits aren't really my thing. I only own one so I think others are more qualified to make recommendations for CAS.