If we define an ace as "someone as good as Kershaw", then Kershaw is the only ace in baseball. Nobody is as good as Kershaw, especially this year.
Weaver has been one of the top 5 most dominant pitchers the past 3 years, but has "struggled" a bit this year even though he is still posting good numbers overall. Not to mention he is in the top 10 amongst active pitchers in just about every category for career numbers including ERA, BAA, WHIP, win/loss %.
Felix Hernandez had 5 starts last year where he gave up 4 earned runs, 3 where he gave up 5 ER, 2 where he gave up 6 ER, and 2 where he gave up 7 ER. That's 12/32 times he gave up 4 or more runs. In the past 4 games he's had a game he gave up 8 and a game he gave up 5, while his other two starts were pretty solid, much like weaver over his last 4.
Yet Felix Hernandez is unanimously considered an ace, but weaver is not. OK