Johan Santana was pretty nasty for several years in the 00s. Was he the best ever, probably not. He did have a great run that would rival previously mentioned player's best 5 seasons.
Kershaw is pretty amazing as well as far as a player we all have been able to see pitch. I hope there is some baseball still yet to see this year
I think Koufax's MLB career might look a little different if he'd had a few years in the minors
Great question. I have enjoyed reading the thread. For a World Series or complete season Koufax is a compelling choice. For a career I still love me some Spahn.