It's hard to choose between Koufax and Spahn. Koufax didn't play long enough and the first half of his career was not even close to what he was in his last six years or so. 1963 Spahn on the other hand probably stuck around two years too long. A couple of the old timers can chime in on this but didn't Warren Spahn lose several years to WWII. If so, that probably robbed him of a couple of very good seasons.
Let me change what I said slightly. Koufax had the best 5 year (or so) peak of any lefty. I always refer to Pedro as the righty Koufax for 1997 - 2002 or so.