Good topic!

It would really depend on what time of the story this question takes place. When Sonny was in his reign/search-and-destroy mission, everyone else seemed small. I would fear Sonny for his aggressive action and lack of war strategy. I would feel as if Sonny was going to take me out next for just being around, though not involved with the hit on the Godfather.

Near the end of the first film, when Michael had the cosa nostra pezzonovante rubbed out during the baptism sequence, that's when I would fear Michael most. Other than that, Vincent seemed like a guy I could get along with.

Don Vito was a character I would fear from the start. The way he worked his way up from an average schnook on the streets to the Godfather we knew him as in Part I. Of course, when his career ended, there would be new people to fear.