The natural litters like World's Best and Swheat Scoop are probably the best. Cats can be very picky about their litter though. I tried to switch my cats to World's Best from a clumping clay litter and they were having none of it, of course that doesn't mean that your's won't make the switch.
Kittens should not use the clumping clay litters though because of their tendency to investigate everything by putting it in their mouth and then swallowing it.