Mine would be either The Godfather Part 1 and/or Part 2, starring as Michael Corleone in both.
Most Helpful Guy
I would be Richard Dreyfus in Close Encounters of the Third Kind.
He plays your basic Everyman character who is visited by aliens and volunteers to go on their spaceship and explore the galaxy, even though time dilation would mean that when he came back all his friends and family would have died or grown very old. I once had a conversation with my friend about if that happened to us, would we go aboard the ship? My friend, who is married with children said yes, but I told him that his children need him and I would stop him from going aboard if I could. He then said that he would make sure I went aboard, because I have no children and I'm a widower. Any rate, it's a great movie.