Void. I shudder to remember Void. You couldn't do anything to the man. He'd just stand there and let whatever you tossed at him sail on past -- or more likely through him -- and although he was silent, I always had the distinct impression that he was laughing at me. I hate Void, and it doesn't help to know that to him I am less than nothing -- a pebble on his path to conquest. Ick.
Then again, Klytus is good at creating bad guys. Devastator is a very serious take on The Tick (deadpan serious, but nigh invulnerable), the only way to knock him down is by moving him, and he had an obscene amount of knockback resistance. Ring Lord wears this ring, on which he has become dependent, that fuels his powers and his armor (his minions have similar, less powerful rings). Personally, White Heat herself is frightened of Whelm, a water elemental working with Void, or she would be if she had ever encountered her. There was this one character, Absolute Zero, who turned another of my PCs, Photon Blue, into an ice statue (yeah, yeah, she finally thawed out), and this absolutely freaked out one of our players, a character named Anaba (7' tall female Native American with Shaman powers of Bear, Deer, and Thunderbird, among others). Anaba was all for killing Absolute Zero, as I recall, but he escaped.
None of his NPCs are indefeatable, they just look it. Ring Lord, for example, after defeating Photon Blue and PsiLord, ended up in the local Stronghold after Photon Blue took strong offense at Ring Lord's putting her buddy PsiLord in the hospital. She took him down single-handed. Before you gasp, you gotta remember that Photon Blue's an absorber, and Ring Lord kept feeding her with energy blasts. Oh, well. Now, whenever he escapes from Stronghold, you can hear him yell across the city: "Where is she?!" PB always says to herself: "Uh oh, he's out again!" and the battle ensues. Personally, I think Ring Lord's hysterical.