So, so many to choose from....
Okay, so for a while a friend of mine ran two Champions campaigns in tandem: Vanguard and Vanguard Europe (this was during the Giffen era of Justice League, not to mention the Iron Age of comics). I ran Vanguard Europe, while playing a character in Vanguard (the ever-lovin' Amazing Man). Vanguard's GM ran a character in Vanguard Europe, a Batman/Moon Knight sort of guy who happened to be team leader (and damn it all, the PC's name escapes me).
Anyways, Vanguard Europe was a pretty powerful group. There was Shocker, the electricity manipulating telepath; super-ninja guy whose coolest aspect was that he took the perk Direction Sense just so he could always leap up and have the moon behind him; there was also the female flying brick too. Plus Moon Knight, whatever his name was.
One session this villain shows up with his super-powered female sidekick and he's packing a ton of gadgets that are home-grown to defeat the villains. For example: an Entangle that absorbed Shocker's electrical Damage Field to then power up a gun that could blast the flying brick. This guy took apart the team in seconds. It was the first villain to whom they had ever really lost. And after defeating them, he unmasked to show this old guy who claims to be Moon Knight from the future, there to stop Vanguard before they destroy the world. (Futuristic versions of existing characters? So early 1990's...)
Flying brick lady managed to escape, but before she can attack the villain Moon Knight attacks her, taking her out. The other players are stunned, but Moon Knight's player says, "I would implicitly trust my future self that I was making the right decision to stop Vanguard."
Which was such a perfect portrayal of his super-intense PC and such a great plot twist all rolled into one.
Sadly, the group was not one that could really differentiate PC from player conflict, and the group never really came back together. Later, the guy playing Moon Knight would again turn on the gaming group mid-session again, only this time it was because he was letting his personal life leak into the game (he had broken up with his girlfriend, also in the group, but they hadn't told us yet), which heralded the end of the group as a whole.