ray lewis or mike singletary.
overall i'd say ray lewis. in his prime there was no player feared in the NFL more than ray lewis. in fact i'd go as far to say not just the most skilled ILB of all time, but most feared of all time by his rivals. thats a bad combination for anyone lining up and the O side of the line.
I'd rank the following as the most feared defensive players of all-time, at their prime, when they played (in no particular order):
Butkus
Dick Nighttrain Lane
Ronnie Lott
Ray Lewis
Joe Greene
Willie Brown
LT
Reggie White and that crazy head slap
Deion Sanders (not because he could hit you, but because he was a ball hawk)
Ed Reed (same as with Sanders - total ball hawk who embarassed quarterbacks and offensive coordinators for a decade)
And Anthony Munoz. No one ever shows love to the hogs. So even though an OL, he makes my list because no one was ever greater.