i think it's hard to decide between gster and speedy...

i think GecTs comments are spot on
and if we could vote 3 times i'd vote in the same order as Coldwave

i think being able to tell someone to be a super villain just from looking at them comes down to perspective, opinion and prejudice ("he has a goatie, so he must be evil" like that)
what might look like a shady fellow to you might be some innocent bystander and vice versa
personally i think the best villains are the ones that look like they aren't evil but have some evil atmosphere about them and the ones who were driven into insanity or evilness and just look crazy.