Well, I don't know whoo makes the best one, but some people think it is best to buy the big name brands. I guess that way you can't go wrong. Like they say when you buy cheap stuff, "you get what you pay for", I think that holds true with the brand name stuff as well, if you want the name, you will pay for it. I have tried to cylinder kits on the ZX, a 50mm and a 47mm, the 47mm came from scrappy dog scooters and was far better than the 50mm one from taiwan.
I think in the end it will be a matter of trying one and see how you go with it. Zombie used to swear by Malossi, but after doing in pistons, he changed to one that I think came from Taiwan,it looked identical to the 50mm kit I had first off.
Still, wait for thebatman, Shaka or zombie to reply, they will give you a better idea. BUT, ask zx50boy,
he is getting 80-85mph from his zx
, he will set you straight.