While yesterday it was easy to winnow the field of hitters down to five names, it's harder to do that with pitchers. There's a definite top tier, with a bunch of guys just behind them. Picking one guy is going to be hard, so let's look at the top tier.
By all accounts, Arrieta had a stellar year, making a huge leap at age 29 and hitting career bests in every category imaginable. He pitched more innings than he ever has before (229), started more games (33), but also got more wins (22). He had the second lowest ERA and WHIP in all of baseball, behind only Zack Greinke. He was just flat out good, and in the most competitive division in baseball. He even pitched a five hit complete game shutout in the Wild Card play-in game against Pittsburgh. (His two other postseason appearances didn't go quite so well, but he almost singlehandedly made them possible. Fun fact: The Phillies faced Arrieta twice this year. The first time he lost en route to the Cubs being no-hit by Cole Hamels. The second time, he made up for the first by going eight innings and allowing six hits and one run. The Phillies lost.
Zack, or as I like to call him, The Guy Who Threw Yasiel Puig's Luggage Onto A Busy Street, had an incredible year. Before 2015, Greinke had been one of the best pitchers in baseball. But this year, his performance singled him out is possibly THE best. And by many metrics, he was. Best ERA (1.66), highest BRef pitcher WAR (9.3), lowest WHIP (.844). He opted out of his deal with the Dodgers, because he obviously thinks he can make more than the obscenely rich Dodgers are already paying him. And he's right.
What more can really be said about Clayton Kershaw? His 2015 campaign continues his five-year reign as one of the most outstanding pitchers in baseball. He's endlessly impressive, incredibly talented and resilient, and in desperate need of a World Series ring. A pitcher hasn't needed a ring this much since Roy Halladay, and he's not even 30 yet. He is just so, so good. His ERA this year was 2.13, which somehow is his highest in three years. He is not human. He is a perfect pitching machine.
Three nearly equal nominees makes for a really hard decision. I don't envy you, dear reader. Best of luck!