I've been using Ever Clean for a while now, and am pretty happy with it. I find that it keeps the smell down, even if the litter is older, and it seems to last pretty long. I've recently tried Precious Cat Ultra, and I found that I loved how it clumps, and that it seems easier to scoop through, but long term it just doesn't keep the oder down, even with daily cleaning of the litter boxes. It's cheaper too, but I seemed to go through alot more of it than the Ever Clean.
Didn't like any of the Arm and Hammer products. They didn't clump, and didn't keep the oder down.