As others have said, there should be a distinction between true freshmen and redshirt freshmen. Redshirt freshmen had an entire year to get acclimated to the college life and game and bulk up for a year. A true freshman doesn't have that luxury.
With that said, I'll go with Jamal Lewis and Eric Berry. Both were ready to play from day 1. If I had to make a choice between those two then I'd go with Jamal. If people remember though, Jamal didn't burst onto the scene until after the Fla game. He wasn't a factor at all. Still can't believe the coaches didn't let him be a factor for that game. They were too worried about his blocking and being able to protect Manning.