And for the short term, I don't know who was more dominating in his prime than Pedro Martinez. I'm not saying he was the best pitcher ever, but at one point he was as good as anyone ever was.
What about Bob Gibson around 1967 and 1968? I weren't alive to see him, but the numbers are ridiculous. A 1.12 ERA with 13 shutouts in 1968? Crazy.
I do remember Orel Hershiser's scoreless inning streak in 1988. Not one of the best ever for his career, but certainly over a chunk of a season he was unbeatable.
I've heard baseball people say that Nolan Ryan threw so many no-hitters because that's all he wanted to do -- so he was unwilling to give up a two-out single in order to save his best stuff for later in the game.
Now, of people I've seen, Clemens and Randy Johnson have been the best for the longest.