Carl McCullough. He was highly rated and didn't live up to it. One could also make an argument for John Clay, but he was pretty productive. Not 5* productive, but still pretty good.
It's very Wisconsin that the black sheep still had 1,200 yards from scrimmage. Granted he was behind two beasts as a freshman and eventually lapped by Dayne
The John Clay thing, that's a hard sell. Here's his run:
Backup who got 880 yards and 9 TDs
BTOPOY with 1,500 and 18 TDs
In his first eight mostly healthy games, got 887 yards and 13 TDs, played bits of three more to get to 1,000 around and ankle and knee
His BTPOY was probably one of the weaker ones, but Clay, kinda like PJ Hill, is a lesson in timing. Flip those last two years, and it feels a whole lot better (Hill was most productive at the start, but his low ceiling left some tired by the end).