3) Anti-Monitor
Perhaps one of the most powerful villains in the DC Cinematic Universe, Anti-Monitor would prove to be a formidable challenge for any superhero. Hellbent on destroying all universes except his own, he (or it?) can’t be reasoned with and only has one goal: take out everything in his path.
Made up of pure energy, his power is unstoppable as he can travel through time, absorb universes and fire bolts of pure energy. The Anti-Monitor doesn’t really have any weaknesses either, and given his reputation as one of the most powerful supervillains, he could factor into the DC Cinematic Universe perfectly as a Thanos-type character.
Speaking of which, although this would never happen, how awesome would it be to see some kind of team-up film between DC and Marvel characters where they have to work together to take down Anti-Monitor?