I am not fan of kits at all. I wish EA never brought them and instead concentrated on SPs and GPs. Kits are cashgrabby and overpriced IMO, containing third of the stuff of SPs for half the price. Therefore I don't consider any kit "must have"
However, there are two I consider good enough to get.
Bust the Dust.
I know this one is controversial, but I like it. It adds some sort of realism to the game with some funny, unrealistic Sims-style additions. I just wish it was better integrated with the rest of the game, but whatever. Still nice.
First Fits Kit.
Because kids never have enough outfits. And these are really cool.
And that's pretty much it for me. If there were sales on Kits, there would be more I could recommend. But for full price, these are pretty much all I can recommend.