Here we go:

Vito- Jack Nicholson
Michael- Gary Coleman (Whatchoo talkin' 'bout Solozzo?)
Sonny-Richard Simmons
Fredo- Steve Buscemi
Connie-Brooke Shields
Kay- John Lithgow
Appolonia- Angelina Jolie
Tom- John Malcovic
Brasi- Brad Garrett (Everybody loves Vito)
Clemenza- Clint Eastwood (Go ahead...make my canoli.)
Tessio- Bob Newhart, even if he's deceased.
Mama- Whoopi Goldberg- she wouldn't have to hand the phone to Sonny...she'd take care of Carlo herself.
Carlo- J-Lo
Johnny Fontaine- Bill Murray
Solozzo- John Lithgow, in a dual role virtuoso performance.
McCluskey- Bruce Willis
Enzo- Barry Williams (Greg Brady)
Barzini-Rupaul
Paulie- Carrot Top