My favorite scene is the original one of Marty going to 1985 but in the second movie when after we see Doc's reaction, which is pretty awesome, we see the other Marty running around the corner. I always loved that scene.
I also quite like it when they go to 2015 in the second film is cool just because of Biff's reaction.
The one's stated above are good too.
"No Marty. You're not thinking 4th dimensionally."
"Yeah, I do that a lot."