Vic Darchinyan when he does his ring walk sometimes has that psychotic smile that kinda creeps me out. In general though as im 6'4 210 - 220 it needs someone with considerable size to intimidate me .... Vitali comes to mind. He has that look that just says he will hurt you and enjoy it.
Definitely these two. These are only two that could knock you out and also look like they'd bum you afterwards.
If you wouldn't know Prime Wlad or Prime Wlad. What would scare you more? A 5,10 with baby voice. Or a 6 ft 6 physical beasts. No offense but his scary factor is mostly because he was a monster in the ring. Outside the ring Larry Holmes is a badder dude if you get him pissy.