My real answer would be that all of the packs are lacking and I didn't really find that any of them added enough items. I own all of the stuff packs and felt a bit ripped off by them.
However, for the sake of diplomacy, if I had to pick some based on concepts, I would say cool kitchen stuff is a great concept. It's very universal. Everyone (or pretty much everyone) has a kitchen in their sim's house. More kitchen stuff is always a good thing. Perfect patio stuff is a good concept for a stuff pack as well for the same reasons.
I think Spooky Stuff is not horribly useful and it should not have been its own pack. They should have done the same thing with spooky stuff that they did with the Christmas stuff, which is make it free.
So, if I had to pick packs that were worth buying, I guess I would go with cool kitchen and perfect patio.
