Overall Haye is the biggest threat. But for the first two rounds Corrie Sanders is as tough a threat as there was for a long time.
Even though Haye doesn't overwealm me with spine tingling chills, he is the best prospect to come along in a few years, and the best bet to beat the Klitchko's. Not to say he will, but he'll be the first in a while to provide a potentially serious challenge.