This list is copied from the post of Freya Coral, somethings I agree so they still stand there. But some things are changed.

Cloud: Hugh Jackman
Barret: Michael Clarke Duncan (Yep, no doubt)
Tifa: (I have absoluty have no idea, but Jessica Alba is no good choice)
Aeris: Cameron Diaz
Red XIII: voice from Sir Anthony Hopkins
Cait Sith: voice from Jimmy Carrey
Vincent: Johnny Depp (Very, very good choice)
Cid: Kiefer Sutherland (good choice, but I'm not sure)
Yuffie: Mandy Moore (Who???)
Sephiroth: Christopher Lee
Rufus: Christopher Walken
Heidegger: John Rhys-Davies
Scarlet: Nicole Kidman
Reeve: Nicolas Cage
Palmer: Danny DeVito
Reno: ???, I dunno
Rude: Vin Diesel (Also a very good choice)
Tseng: ??? =, maybe Jackie Chan
Don Corneo: Jack Nicholson