Neither film should've made it this far. But if I had to choose between the two, I would go with Back To The Future.
Sorry, but I still believe Nolan's Batman films wouldn't receive as much praise, if Schumacher's turds never existed (Batman Forever, Batman & Robin). Plus, a lot of people are still caught up in the "what could've been" stuff with The Joker. Heath Ledger's untimely death gives this film a boost, because people will always look at The Joker performance, and wonder about the great career that could've been, if Heath was still alive.
Back To The Future probably won't win this round, but I'm still hoping for Marty and Doc to take home the grand prize. Twenty Seven years later, and Back To The Future is STILL praised as a timeless classic.
The Dark Knight was just released five years ago, and you can make strong arguments for Batman Begins and The Dark Knight Rises being the better films in Nolan's Batman series. Let's give The Dark Knight some more time, and let's see how it stands up against other superhero films over the next ten or twenty years. But no, Dark Knight fans already praise this film as the gold standard for superhero films.