Martian Manhunter/YMMV


 * Complete Monster: Commander Blanx, the Pole Dweller (White Martian) responsible for the planet's extinction in pre-Crisis continuity. Disgusted with Mars' pacifistic ways, Blanx restarted a race war after a millenium of peace, took over the planet, destroyed J'onn's reputation on Earth, and discovering that wasn't enough set Mars on fire. Why? Because a group of aliens had offered to buy Mars from him, and he couldn't sell if he wasn't the last surviving Martian. Completely apathetic about the planet's fate, Blanx engineered a genocide against both J'onn's people and his own, in exchange for money.
 * Post-Crisis, Malefic may also count. Like Blanx, he's responsible for the destruction of life on Mars. Unlike Blanx, he runs on seething hatred of all things Martian, and has a penchant for Mind Rape, torture, and Kick the Dog style murder as well. Also unlike Blanx, he has something approaching a Freudian Excuse: his mother disowned him at birth after seeing a prophecy about how evil he would be, and he was more or less raised believing he would be a monster. Moreover, he's quite obviously completely, and utterly insane.
 * Crowning Moment of Funny: Trying to disguise himself, J'onn shapeshifts into a Japanese girl. Named Rei Hino. This successfully fools Clark Kent... But Bruce Wayne sees right through it - because the name was too obvious.
 * Uncanny Valley: J'onn's almost-but-not-quite human shape sometimes disconcerts people. (And this was the form he created to avoid scaring people!)