Mark turning the tables on master manipulator Dyson Frost (who had apparently predicted his every move) by spitting siphoned gas into Dyson Frost's face and then proceeding to put him in a headlock.
Demetri's Big Damn Heroes moment when he rams Raynaud's car to prevent him from murdering Celia. Of course it doesn't totally work out for Celia, but still...
Mark taking down several opponents in the finale despite being outnumbered and having inferior weaponry. He then risks his life by staying behind to contact Stan so that he can warn the world of the incoming blackout to minimize casualties, and makes one final charge to the window to escape despite having practically no chance of surviving. Based on Charlie's flashforward, it's implied he somehow made it.
"You have to learn to lock the beast away; it has no place in society. But you know what? Today, I'm going to unlock the cage. Today, I'm going to let it out." Commence beatdown to the tune of Black Betty.
Simon killing Flosso. "I never get pushed around. And I always get even."