I love Donnie Darko and would back it against a lot of films in the tournament, but Back to the Future is on a whole other level.
Donnie Darko is a very deep and mysterious film. Not many movies you'll ever watch will make you sit and reflect afterwards like Donnie Darko will.
Back to the Future is just the opposite... pure popcorn flick that you just sit back and enjoy.
The cool, pretentious side wants of me to vote for Donnie Darko (especially when you look at my reasoning for voting The Matrix over Scarface, lol), but I just can't do it. I hated Donnie Darko the first time I saw it when I was a teenager. I love it now, but I realize it's not for everybody. Back to the Future on the other hand... you won't find many people who don't absolutely adore this film, and rightfully so. It's one of the funnest movies you'll ever have the chance to sit through, and it only gets more fun with each viewing. Not many other films can say the same, including Donnie Darko.