Fedor, Cro Cop and Wandi.
Cro Cop & Wandi are easy to understand why.
But Fedor, a LOT of fighters described his demeanor was so cold that people got afraid of it.
As for today, I'd say it was Anderson Silva (pre-Weidman) and Jon (pre-Gus).
Those two were unstoppable and unpredictable.
Now they look more "human" and while I do think that Jon can go back to that aura (if he beats Glover & Gus), with Silva it's probably lost.
I would add Aldo (pre UFC) and Barao.
But Aldo is losing it (if he keeps playing safe to a decision) and Barao does not get the respect he deserves (but probably will if he beats Faber once more)