In general, in the past 10-15 years, Leaf was the biggest bust I have seen.
For the Bills, Mike Williams or Erik Flowers. Can't decide between those 2.
Also, Andre Ware was pretty bad too.
What about that whole RB crop, Rashaan S'alaam, Ki-Jana Karter and Tyrone Wheatley. THey were all hyped to be great, and they all went belly up.