It very's. You can talk about Aces in the league or the Ace of a team but just because he is the Ace of his team doesn't make him in Ace in the league. An Ace to me is a guy you expect to win every time out, a shut down guy, someone who ends losing streaks, a horse that sort of thing. In the league right now imo are Aces(in no particular order),
Verlander
Kershaw
Cain
Felix
CC(was prior to this season for a long stretch)
Price
Weaver
Hammels
That's it. The next group are guys with Ace type stuff and have shown stretches of dominance but still have inconsistency's/ injury issues the keep them out of my top group, this group are guys like,
Strasburg(he could arguably be in the Aces group, I just held him off because of his DL history)
Grienke
Gio
Lester
Darvish
Lee
Cueto
I think Buccholz is almost in this group but he has to show he can give you those kind of innings year in and year out, I think you could add him in the next group with guys like Shields, Wilson, Latos, Bailey, Johnson of the world.
I'm probably missing good SP on these lists but they aren't complete just used to give an idea of how I judge what an Ace is.