Hindsight is always 20/20. I think it's only fair to judge bad picks if they also looked bad the day they were made; 'reaches' that didn't pan out. Even the infamous Mike WIlliams pick seemed like a good decision on draft day.
The most egregious examples I can think of in recent Bills history were Aaron Maybin, Donte Whitner, CJ Spiller, Leodis McKelvin, and Sammy Watkins. None of these passed the smell test on draft day. And to be fair, only one of them (Maybin) was a complete and total bust - meaning he never provided anything of value to the team whatsoever. The rest were certainly over-drafted, but they at least contributed something on the field, and/or brought something back to the team in a trade later on.