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Marc S.hard to say...it sounds like you are almost biasing your answer against relievers --- and there have been numerous times that Closers have won the Cy Young award, most of the time, deservedly so.
I think if you adjust ERA (for park factors, etc.) it is a fine starting metric. I also think that fielding should play a stronger role in the award. For example, Greg Maddux has won numerous Gold Gloves and Cy Young Awards. But if you use ERA only as your primary starting point, you are almost implicitly biasing against someone like Maddux who would give up more (unearned) runs as a consequence of his crappy fielding.
I think Win Percentage compared to Team Win Percentage is a pretty good metric, too. For example, this would hugely highlight the dominance that some pitchers have had over time. Steve Carlton's 1972 season is an excellent example of this. A phenomenal winning percentage on an absolutely horrific team.
I also really like WHIP as a metric, and it covers a lot of things, like OBP, etc.