Kick-Ass defending that guy seemed like A Crowning Moment of Awesome, but what really kind of hit me was the way the guy said "Thank You". I'm sure that's what made fighting worth while for Kick Ass.
Hit-Girl's final moment with Big Daddy as he dies.
Their whole relationship throughout the movie (and comic) is adorable in a very Addams Family kind of way.
The music during the gun battle just prior as well - the action is her dishing out awesome, but the music is channeling Big Daddy's pride watching his daughter slaughter everyone.
Not to mention, even as Big Daddy is burning to death, he keeps shouting encouragement and advice to Hit-Girl.
Big Daddy believes he is about to die having failed his daughter. But when he's set alight and sees her alive before him; he has no thought of his own pain. And those three words.
Damon Macready: TAKE COVER, CHILD!
In the film, Katie forgiving Dave for lying about his sexuality and letting him spend the night.
Even though Frank is a cold-blood killer, there are several moments where you can tell he really loves and cares about his son. Like when he thinks Chris died in the lumber mill and their reunion hug. Just look at the relief on Frank's face when he sees his son is alive.
"Listen, my son's waiting in the car and wants to see a movie. I'm not going to disappoint him."
When Kick-Ass meets Big Daddy in person, the first thing that he says is "We like you, but we don't trust you." Which is rare because the only people he expressed any affection for is Hit-Girl and Marcus. It's further lampshaded by Hit-Girl later on after Kick-ass saves her and kills Frank that he would have been proud of them both, rather than just her.
In the final issue, Dave finally gets his happy ending.
Chris and Mindy's mothers bonding over their children being costumed killers. You can also theorize that Mrs. Genovese learned that Chris died to save Mindy's life, which added to their friendship even more.