Joe Louis
Rocky Marciano
Ray Robinson
Henry Armstrong
Roberto Duran
Ray Leonard
Marvin Hagler
Tommy Hearns
Carmen Basillio
Gene Tunney
Jack Dempsey
Joe Frazier
Ali
George Foreman

God I could go on for days about great fighters but guys fighting right now that can be great are:
Jermain Taylor
Miguel Cotto
Joe Calzaghe
Chad Dawson
Floyd Mayweather Jr (he's still in the relm of Very Good IMO)
Manny Pacquiao

It's harder for Heavyweights as they have a much more limited group of opposition and they have to do more to be considered great as the greats in that division are really GREAT

That being said Wlad and Vitali have shots at being greats but time's a tickin by pretty fast for them