The Time Trapper (Legion of Superheroes)
Mordu(Legion of Superheroes)
Krona (Green Lantern)
The Time Trapper (Legion of Superheroes)
Mordu(Legion of Superheroes)
Krona (Green Lantern)
The Crime Syndicate (Earth Two), Mandrakk (Final Crisis), Reverse Flash (Flash), and Barbatos (Metal). I knew Mandrakk and barbatos is only a plot-convenience villain, but I think they still had hidden untapped potential.
The Controllers shouldn't be overlooked. They don't like you, they'll cook up a Sun Eater and let it run amok.
Does Epoch with his Revision Mechanic count ? After all, it's what can potentially allow Darkseid to rewrite the whole history of the Multiverse.
Perpetua is also not half-bad. She could be used better, but she got a lot of potential, I think.