Lots of great names mentioned here, I'd add a few that really stood out to me over the years:
Aaron Hunt (Tech) and more recently, Will McDonald (Iowa State) - Hunt was still the lone reigning all-time sack leader in the Big 12 conference, had a inhuman knack for getting to the QB, until McDonald tied his record last year I think?
Derrick Johnson (Texas), Roy Williams (Texas) - Kind of fit in the same category, even though they played different positions. What monsters those guys were, it was very rare to see players at their size and speed in their positions in college at the time.
Tommie Frazier/Eric Crouch (Nebraska) - Both terrifying to play against for basically the same exact reason
Terence Newman (Kansas State) - Everything you'd ever want in a DB ever. Nothing more to say.
Roy Williams (Oklahoma) - Just a smart and extremely intimidating defender all around, one of the all timers
Woods brothers (Oklahoma State) - OSU legends for a reason, helped lay the foundation for their program to build
And to dip straight in the clown bag:
LaQuan McGowan (Baylor) - 400lb tight end, what else is there to say? WTF?