I voted for the Allegro -- not because it's my favorite, but because it's the only movement I know.
When I was in college, I heard this piece every single school day for the entire academic year. It's what the Form & Analysis professor played to demonstrate the fundamentals: how to analyze with your ears and not with your eyes. Its simple Classical Sonata Allegro form was the only one we studied. Not rondo, or binary form or rounded binary form and not even Romantic Sonata Allegro (i.e. theme groups.) I once remarked to a professor afterward that if I hear that theme when I die, I'll know I've gone to hell. At least that's how I felt at the time.
Now that I've been away from college for some years, I can stand to listen to it again, and this time venture forth and listen to all four movements. (Well, OK, maybe just the other three.) At any rate, thanks for the post!