It's hard to pick between those first two; masterpieces, both. However, if you put a gun to my head, Luca Brazi style, I guess I would pick the first one, by a very slight lead. But that takes nothing away from the brilliant sequel.
The third one has its moments, but isn't in the same league as the first two...
I first saw The Godfather I and II all swirled together as The Godfather Saga, so I & II are equally great to me even as I watch them now as individual films. Maybe II, with its past and present contrasts and ambitious scope, is the slightly better movie, but the first one is such perfection.
The Godfather III is also very good. I'm even OK with Sophia's lambasted performance; her amateur acting fits well with the naivete of her character.
Original is the best henry it had an amazing cast complimenting each-other its possibly the darkest script, oh horsey. Just watching Pacino gradually take the reigns from Brando was unpredictable yet very gratifying & his intelligence & well-fare at the hospital attempt the restaurant slayings was heroic.. as lead participant to both nasty events no-one else could stabilize the Don more nimbly, TGF & II are Pacino's career bests.
De Niro in II - one of his best performances, Vito was poor a quiet character also abit crafty minded if needed - but when Vito finally got poked-too-death by Fanucci the story unravelled in great proportions the under-world gradually formed & built a gangster empire, Michael against Roth was also a great-side notion that finally broke the family saga maybe the most callous piece in the franchise when Fredo is sadly taken.