18. Johnny Manziel (2014)
Even during his highly scrutinized and controversy-filled redshirt sophomore year at Texas A&M, Johnny Manziel went from a frenetic but magical Heisman Trophy-winning quarterback to a legitimate NFL prospect at the position. At his best, he was the first quarterback in NCAA history to pass for 3,000 yards and run for 1,000 yards in a single season.
At his worst, he was a constant headache for his coaches, as his off-the-field exploits made even more headlines than his antics on the field (like taunting opponents). The fact that he only played two years of college football also gave some teams pause.