The idea of an ace in baseball is sort of silly, or at the very least, somewhat nebulous. There are thirty teams, so are there thirty aces? If so, is each team's best pitcher an ace, or are the top thirty starting pitchers the aces, and some teams have more than one while other teams have none? Whatever an ace is, I don't think there are thirty of them. Forced to come up with some sort of determining criteria, I suspect I'd end up with something like 15-20 aces, and a very small number of teams would have more than one.
If you're too stuck on the idea of what an ace is in baseball, perhaps the more helpful question would be...
Who is the Indians' best starting pitcher?
Corey Kluber deservedly won the American League Cy Young Award in 2014, and was the easy answer to this question as recently as two months ago. Things have been different during the second half though, because Carlos Carrasco has been far and away the best results of any starter on the team during that time, with a league-leading 2.49 ERA and a second-best 2.77 FIP and 11.07 K/9.
Are two months enough to change the answer to the question? Not exactly, but Carrasco has been so good during that time (and Kluber, in part due to an injury, has been good, but not great) that his numbers over longer spans of time are catching up to and in some cases surpassing Kluber's.
Carrasco 2015: 3.44 ERA, 124 ERA+, 2.79 FIP, 10.51 K/9, 2.04 BB/9, 0.85 HR/9, 180.2 IP, 4.2 bWAR, 4.8 fWAR
Kluber 2015: 3.62 ERA, 118 ERA+, 3.02 FIP, 9.93 K/9, 1.81 BB/9, 0.93 HR/9, 214.0 IP, 3.8 bWAR, 5.2 fWAR
Carrasco has better rate stats, but Kluber deserves credit for having thrown more than 30 additional innings. Balancing those two things, the two of them are awfully close.
I don't love that so many around baseball (especially among average fans) ignore almost everything but the current season in deciding how good a player is. Single-season numbers are good for end-of-year awards, but in assessing value, at least one additional year should be considered. What of we combine the last two years?
Carrasco 2014-15: 3.06 ERA, 132 ERA+, 2.67 FIP, 10.04 K/9, 2.00 BB/9, 0.69 HR/9, 314.2 IP, 7.9 bWAR, 8.0 fWAR
Kluber 2014-15: 3.00 ERA, 132 ERA+, 2.64 FIP, 10.11 K/9, 1.88 BB/9, 0.72 HR/9, 449.2 IP, 11.2 bWAR, 12.6 fWAR
When you look at the last two years together, Kluber pulls way ahead in both version of WAR, because he's thrown far more innings than Carrasco during that time. That said, Carrasco still holds his own against Kluber in terms of their rates. Frankly, you could hardly find more similar pitchers in terms of their rates. ERA, FIP, strikeouts, walks, home runs... everything is practically identical.
Ultimately, they both deserve to be considered aces, giving the Indians a huge a big advantage over most other teams. As to which one of them is better, it's in the eye of the beholder. For me, Kluber wins out because he's done everything Carrasco has done, but in 40% more innings. The more weight you add to more recent production though, and the stronger Carrasco's case becomes.