I'm using World's best but having second thoughts, as although it's very very good it's hellish expensive, and four of our eight cats now use it so I'm getting though 6 litres a week.
I've recently tried Sanicat Gold from Tescos, won't use it again. Gritty stuff all over the house, it sets like concrete and isn't flushable. I like to compost the weewee clumps and flush the poo so need one that's organic based, not clay or crystals.
Some members use chick-crumb as a good alternative to Worlds Best, as it's made from the same stuff, but there's no supplier near me in NE London so haven't been able to check this out myself yet.
I have two trays just for wee - the upstairs trays get used as pottys in the night (by the cats not us ! ) - and these are lined with free newspapers which I recycle after use, and of course, they are free!