I think Cliff Lee is a great pitcher (one of my favorites to watch), and don't get me wrong...but if you want to be the best EVER at something you have to win something. At this point he has not won a game that clinched either a league pennant or World Series. Until you win something you can't be the greatest ever at it.
Lee will move into a league with guys like Ford, Koufax, Gibson, Jack Morris (who was awesome in the post season until he got old), Schilling (even though I hate him I will give him his due) when he clinches something more than a division series.