Donovan McNabb, and get out of my head! I was going to pick him for this list too.
McNabb was a guy with lots of physical talent but dumb as a stump on the mental side of things. When he finally gets to the Super Bowl he parties hardy the night before the game and gets so dehydrated that in the second half of the game he was so worn out he couldn't even run a two minute offense because he was sucking wind...