That's a good choice as well. I do think we'd need to establish some rules for consistency, though; for example, Dickey has had two great years framed by virtual nothingness on one side and a bad start on the other. Part of the problem with defining "aces" is that by the time you're convinced that they are an ace and can reproduce those ace-like results reliably, they are already past their prime and beginning decline. Very, very few pitchers are an "Ace" for, say, 5+ years, so that you can soundly establish their role as an "Ace" and have them keep performing at that level for several years after that.
I'd say pretty much anyone who performs at that level would be at or near hall of fame quality, basically.