I've used pretty much all of the commercially available kits and they're all pretty damn good these days.
CCM is the benchmark standard, but as others have said pretty much anything with an additional stablizing rod will feel nice. Slice, CheckIt, etc are all pretty good kits. I haven't used the Inception kits, but I hear good things. Like Jkoselka said a new spring kit will have a big effect on smoothness/stroke.
I may be putting up a CheckIt pump kit soon, if I do you can have first dibs.