All three of the Godfather movies were great. The original was a classic, but I always think more of Marlon Brando's performance in that one than Pacino's.
Godfather III was my second favorite movie in the trilogy. Pacino was great in that one as well.
Most people seem to say they liked Godfather II the best, but I didn't like all the jumping around between Michael's and Vito's lives.
My favorite Al Pacino performance ever was in Scent of a Woman. That doesn't answer the question about crime classics though, so I'll say his best performance in a crime classic was in Scarface.