My favourite is Fire Lord for the same reasons that @Stoax said in the first post.
A close second would be Corroder, probably due to the fact the is probably the most effective non-leader villains to stand his own against the heroes (heck he brought Bulk, Stringer, and Surge down to their knees back in "Core Crisis").
Plus, I admire his sheer unrelenting drive to straight-up destroy the heroes, even when they put up their "Hero Cell" (force field) Corroder goes into full rage mode and tries crushing them with girders, and drowning them in acid ("I don't care if it takes all day...I am going to DESTROY you!"). He only retreated when faced with a whole army of heroes (a bluff set by Furno). Gotta love a villain with determination.
It was a shame that he was defeated so easily in the last episode along with all the other "extremely dangerous" villains.