If what I hear is correct, no pitcher was ever as dominating as Koufax was in his prime. So if we are doing this based on who had the best ability to pitch, Koufax. But hell, if you base this list on what someone does when they are at their best then you would have people saying Ron Guidry or Mark Fydrich are the best of all time.
If the question is who had the best career, I say Young or Johnson.
I have always been a big Nolan Ryan fan. When I was young the name Nolan Ryan was connnected with greatest pitcher ever in my mind. As I started reading and just learning about othe pitchers I learned that Ryan was not the greatest pitcher ever, but possibly the ballsiest. Ryan didn't care about walks, he just didn't want to give up a hit, on a 3-0 or 3-1 count Ryan would fire in a curve ball. simply amazing, not to mention the fact the guy threw 100+ mph.
Had Ryan played on better teams he would have won 400 games IMO. But he got his ring in '69.
There are a lot of guys who probably deserve to be considered, but it would be impossible to make a long enough list. Pedro, Randy, and Maddux are just as dominating as Roger, their careers have just been shorter.
Tom Seaver is also one guy who probably deserves more credit than he gets. As does Gaylord Perry, even if he threw spitballs, it still takes a hell of a pitcher to win 300 games.