There are so many good Michael scenes. It would be hard to chose.
In Godfather I: When he is in the hospital and discovers his father is alone, he calls Sonny and says, "Papa's here alone. I won't panick!" and he convinces the nurse, who is very reluctant at first, to move his father to a safer location.
In Godfather II: The whole movie.

Honestly, the ENTIRE movie. I love Al in Part II, it is my favorite.
In Godfather III: when he dances with his daughter Mary at the beginning might be a good choice (not many favorite scenes of Michael in Part III for me).