My favorite is Part II. The tremendous effort put into GF 1 was exceeded here, at least IMO. LOVED Vito's prequel story. And of course Robert De Niro is phenominal.
The original is my next favorite (Marlon Brando and Pacino. Incomparable actors.)
Part III is a very good film IMO, it just has a terrible reputation because of Sofia Coppola. To be honest, I didn't think she was that horrible. Not a great acting job, but competent. And Al Pacino was perfect as always.

"It rubs the lotion on its skin or else it gets the hose again." - Jame Gumb, The Silence of the Lambs.

"I'm sick of carrying guns and beating up women."- James Cagney