Shawn Bryson. Not only was the guy was lightning quick, but he was an EXCELLENT blocker, and a GREAT team guy. He could've played tailback, but chose to be a team player and sacrificed ALOT by playing fullback.
The other picks I like in this thread are Travis Stephens and Fred White.
Another good one is Eric Westmorland. Lots of guys were underrated when they were here during the Peyton Manning years and the national championship season. Main reason they were underrated was because they were overlooked by all the other great players on those teams.