Tough call.
Was going to ask this question as well a couple of days ago haha.
Paul Skenes has received more media attention in my view, whereas Jackson Merrill hasn't got the same fanfare, though quietly has been really good posting a 4.2 fWAR season thus far and helping the Padres make the Wild Card. It's either one of those two, but since Merrill is an everyday player and has been really good, would go with him.
As for the AL, it's not so clear cut. Cowser has been a solid piece for the O's, though offensively he hasn't put up eye popping numbers. He leads all the candidates in terms of WAR. As for Wells, playing behind the plate on a contending team is definitely an advantage. Gil's numbers have slowed down since his hot start. Mason Miller has been electric and one of the best closers/relievers in baseball this season. I know he's just a closer, but his numbers stand out in comparison more to the other candidates. We've seen closers win ROY before, so wouldn't surprise me if Mason Miller gets some serious consideration.