The concept of an ace comes up a lot in baseball conversations, usually without a specific definition. In scouting, it comes up just as often and has a bit more of a specific meaning, but it can vary from person to person. The basic idea, in a scouting sense, is that an ace is part of the top tier of starting pitchers and that this tier is more than five or six pitchers but less than 15 -- with most scouts and executives putting it at eight to 12.
I've never heard it described this way, but it makes sense to think of it as basically one ace per playoff team, excepting last year's 16-team free-for-all. It worked out that the playoff format currently includes 10 teams and the list right now has 10 pitchers -- but it won't always be that way.
This ranking, which we will regularly update to keep track of what the top of the big leagues looks like to evaluators, is projecting a bit. But since it's for right now and the rest of this season, it's basically evaluating what a guy can do at the moment -- not whether he'll make the leap in the years ahead. There's a big example that I'll get into below of an injured ace, and he won't qualify for this set of the rankings, but I'll keep his seat warm. As with almost all of my rankings, this is my opinion but has a lot of industry views folded into it.