Regarding the Godfather I vs II debate. While I think both are terrific, I'm partial to the first one.
In terms of performances and overall filmmaking, both are amazing, so it's impossible to separate them in those terms. But in terms of narrative flow, the first one is superior to me and about as perfect as it gets. Not that the second one is bad in that regard, but intersecting two different storylines always causes a bit of a stop-start feeling in the narrative flow, which is something I find distracting in movies.
Thematically, I find part II to be more complex and darker, but it does get a bit heavy and depressing towards the end. While I found the simpler descent in the first one a little more satisfying. Still, they both are two of the best movies I have ever seen and complement each other perfectly.