Assuming that we can play God and make that judgement:

Vito: Hell for murder
Mama: Purgatory, although she didn't commit any crimes that we know of, she knew of them and didn't tell the authorities.
Kay: Hell for the abortion and the bad jewelry.
Michael: Hell for murder
Sonny: Hell for murder and adultery and numerous crimes.
Fredo: Hell for adultery
Connie: Hell for murder and adultery
Apollonia: Heaven
Don Ciccio: Hell for murder and a bad moustache
Hyman Roth: Hell for stealing, lying, we don't know if murdered anyone directly.
Frankie Pentangelli: Hell for all the things Tom Hagen said they got him on and suicide.
Vincent: Hell for murder and bad taste in women.
Mary: Actors purgatory.
Khartoum: Horse heaven, he never did get to stud
Woltz: The deepest pits of Hell for cavorting with minors. What a pig.