I chose Part 2, because of Part 1. I know that may sound convoluted, but hear me out. Seeing Michael's rise in Part 1 only made Part 2 greater because you got to see Vito's rise to power in Part 2, coupled with Michael's struggles as the head of the family.
Seeing the different philosophical approach of Vito compared to Michael is part of what makes Part 2 so great. Vito made a concerted effort to be respected. Michael made a concerted effort to be feared. So in the end, while Vito is able to enjoy the gravitas of being "Don Corleone", he's also able to enjoy the thing that's most important to him, which is family. The way part 2 ends, with Michael alone with his thoughts, is a representation of how alienated he's become with the weight of carrying the Corleone legacy. That, to me, makes Part 2 the better flick, but it's really a matter of nuance.
This is still one of my favorite images from a film EVER: