What are great rotors for mountbike? I have Avid rotors and they loud. Any advice?

Brake Authority.

What brake and rotor do you have?
Is the brake aligned and setup properly?
Have the brakes always been noisy?
Have the rotors or pads had any contaminates like lubricants or cleaning agent on them?

^ What he said. I've got Avid G2s and they're quiet. Maybe a squeal once in a while(mostly a brief noise at low speed when releasing the brake), but they're a lot better than other brakes I've heard on the trail, and better than the rotors I had before I got the G2s. I'd give them a good recommendation. Whatever you do have, make sure you answer the questions above before getting new ones. And make sure your pads are nice and clean and not glazed over. Look into brake pad baking(in the oven, like a cake) if you have potential pad contamination. It really does work.