The Godfather part 1 is the most fantastically directed, excellently written and incredibly acted film of all time, in my opinion. Marlon Brando really protrays the Itallian elements of being part of a family, both biologically and in terms of the mafia. AL Pacino is at his best, managing to portray two different personalities (Anti-mafia and the Don) prefectly. For me the film is about family, although it is obviously very much about the mafia. The way family values are shown, and the heartbreak on Don Corleone's face when he hears about the death of his son Santino. Just brilliant.
Part 2 is equally as good, some say better, however I feel that part 1 is the better of the two. Robert DeNiro's acting is flawless, as is Pacino's, however the storyline, in my opinion, doesn't match that of the first, yes, it's brilliant, but I don't understand why people say it is better than the prequel.
Part 3... terrible.