I liked him, but in truth, I preferred the supporting cast. And Brando stole the movie. The scene just before he dies, where he's outlining who'll betray Michael, is a work of art.
And Duvall was great, as was Abe Vigoda. Plenty of good heavies in this flick, too.
But Pacino did OK. He does menacing, very well.
What made Michael stand out, was his transformation during the course of The Godfather, how bit by bit he was pulled into that world that in the beginning, he wanted no part of.