The first movie is very, very good. If it has a fault, it's that the central character doesn't quite come into focus. it should be Al Pacino's movie, but Marlon Brando steals it. You could argue that Brando's mannered performance knocks the film a little off balance, but it's hugely enjoyable, so who cares?
The second movie is note perfect. Pacino comes into his own. De Niro plays Brando better than Brando does. And lets not forget John Cazale who makes something wonderful out of poor, sidelined Fredo. It's probably the deepest, most Shakespearian film ever to be made on a Hollywood blockbuster budget.
The third movie isn't nearly as bad as people say. It's the most ambitious of the three- and maybe that's why it wobbles. The script isn't good enough. "This Pope has powerful enemies; We may not be in time to save him." Yeah right! But it's got some great set-pieces and if it's unbelievable, so was the (true) story of Vatican skullduggery on which it's based.
They're three very different films. Will Hollywood ever again bring itself to make an epic as intelligent and morally complex as this? Probably not.