I use the multi cat clumping formula by Tidy Cats. We have good luck with it. Basically, I like anythign that clumps because the urine will smell so bad if left at the bottom of the box.

The best thing we ever did, though, was install a pet door. We put the litter boxes in a closet in the basement, then installed a pet door. The cats love it- they play with the pet door for hours. They figured it out immediately and have no problems with it. Now, there is no smell in the house or the basement, and the mess is kept hidden. Plus, I think the boys like their privacy.