silica is the best but it's nasty dust too, i mean, for soaking up the smell of pee... but yeah, i found the best was the paper/wood based stuff

also, if your cats are females you have a lot more problems with toilet, you have to really keep the tray clean or they won't have it... the females refuse to pee if they can smell pee, they will go to the carpets and rugs and even beds instead

so it probably helps also to make sure where you site their trays that they are well ventilated areas to help keep them from deciding to pee elsewhere