From what I understand, paper based litter is best! Many cat litters are too dusty for rats, even some that claim to be "dust free". I think a lot of people use the brand Yesterday's News! I also have used care fresh but that's just because I use fleece bedding, and had a bunch of leftover care fresh.
If I use litter I just use the small pet litter from pet stores! It works well, however some of my rats like to have "litter parties" where they decide to throw the litter bedding as far as they can across the room... For this reason most of my litter boxes don't have bedding in them, I either clip fleece to them or leave them empty!