Most Helpful Guy
It's entirely subjective because everyone's favorite movies can be considered "greatest movie of all time" to them.
I'd have to go with the following dozen or so movies that I can remember from the top of my head:
A. I. Artificial Intelligence
Terminator (just the first 2)
The Transformers: The Movie (the 1986 animated original)
The Iron Giant
Army of Darkness (and it's prequels Evil Dead and Evil Dead II)
And I'm sure there will be more I didn't list.
And I can't really pick a single one out of those as the "greatest" movie of all time, as it really depends on what I really want to watch.