From what I've seen, the best is whatever works best for you and your rats. Some people love one, and others didn't like it. Sometimes, you have to experiment.
As for safe, just off the top of my head (so I may forgot something):
Aspen, Carefresh (regular or ultra), Yesterday's News (not scented... The cat litter is often cheaper than the rabbit stuff, and it's the same exact thing), alfalfa pellets (rabbit food, yep), wood stove pellets, eco-bedding (though it doesn't do anything for odor and works best mixed with something - but they like playing with it!), Harlan Teklad beddings... I'm sure I'm forgetting something.
What NOT to use (not a complete list!):
Cedar, pine, most cat litter (newspaper based lile YN is fine).