I'm not much of a fan of the term ace because I think the best pitcher on every team is the team ace so there are 30 of them, but going by the definition you are using I wouldn't say the stuff has to be great but they have to go deep into games and pitch well on a consistent basis and do it for multiple seasons. So someone like Kazmir or Harden wouldn't be an ace, they just leave the game too early too often. Halladay, Lincecum, Sabathia, Peavy, Santana, Hamels are the only sure things I can think of. Peavy being a little questionable because of the park he plays in. Haren, Beckett, Hernandez, Oswalt could all make a strong case as well. Guys like Greinke, Gallardo, Johnson could become aces with a little longer track record. I forgot Webb in my initial post who would go in the ace list if he weren't injured. Lackey would probably go into the has a strong case if he were healthy as would Sheets. Not sure what to do with the injured guys in the discussion.