Who is the Cubs ace?

When the Cubs signed Jon Lester, he was immediately penciled in as the Opening Day starter. But is he the best pitcher on the staff?
The six years and the $155 million dollars the Cubs invested in Lester would seem to indicate he's the ace of the staff, but if you ask Lester, he may just name someone else.
Is @JArrieta34 good?! #Ace
— Jon Lester (@JLester34) May 13, 2015
Jake Arrieta had his coming out party in 2014 when he went 10-5 with a 2.53 ERA in 25 starts. And so far in 2015 he's proving that last season was no fluke.
The 29-year old is 4-3 with an ERA of 3.00 through his first seven starts and on Tuesday night he showed how dominant he can be when everything is clicking.
Arrieta tossed eight innings against the Mets on Tuesday night, allowing just one run while striking out 10.
