I admittedly don't get the hate for Part III. It's poetic justice. And Pacino is actually emotional in it as opposed to stoic and calculating. It's like he's struggling against time and fate.
I'm indifferent to Michael personally in Part I. He rises to power in a system that's basically Medieval.
Part II is a meandering, unfocused wilderness of a movie, but it's finally, ultimately, at long, long, long last rewarding. I do find him basically repulsive by the end, though.
I've always said that III has a great, appropriate ending. But other than that, the movie is a catastrophe. I'll watch it from time to time because we all need those horribly cheesy comedies every so often.