Depends on the person. Lance loved high cadence and short cranks. Jan Ulrich mashed massive gears.
Personally, I find that I use 90-100rpm depending on gradient. Anything over 100rpm puts too much requirements on the lungs. Anything below 90rpm makes the legs hurt more than the lungs.
Basically, you want to get your HR as high as you can hold it, then have a gear that makes your legs hurt, but just below your pain threshold.