I've been thinking about this for a while and I'm torn. There are problems with it. Take De Niro, for example. Based on his filmography alone he should be near the top of the list. There's no doubt he has been in some great films, Heat, Casino, Taxi Driver and the Godfather to name a few. But does that make him a great actor? I'm not so sure.
I'm going to steal an old, old, post from Maddox about a tribute to real men. Don't take it too seriously, but Peck was seriously baller in To Kill a Mockingbird.
http://www.thebestpageintheuniverse.net/images/peck_theman.jpg
http://www.thebestpageintheuniverse.net/images/peck_driving.jpg
I'm not enough of a film buff (modern or historical) to make any kind of educated guess as to other truly great actors.