Obviously for me it's The Hunger Games.
-First film of a YA trilogy, and just knocks it out of the park. Not only beating the rest OW, but just completely destroying them, enough so to be close to even TDK's OW.
-It knocked the box office out of the park with probably the most weirdest, darkest, twisted story people could hear about, kids killing each other on national television. Definitely not the storyline you look at and go "oh man, that's gonna go big!"
-It took and still has the top biggest opening in March (again, it opened in March and not in the summer where usually big blockbusters open) ever, and did for awhile have the biggest opening for a non-sequel, now in second place.
- It had no 3D.
- It went from being an unknown months away when the trailer was released in November, to being one of the biggest blockbuster hits from the amount of hype that came from nowhere. It was never expected to do even half of what it did OW (unlike TDK and Avengers).
-No one would have ever guessed it to make over 400M DOM.
-Oh. And of course, the birth of a legend named JLaw.