I think the answer would have to be, almost unequivocally, Josh Beckett. The other options are an obese 69 year-old alcoholic, or a comically fat cat, who has never actually existed. I would be surprised if the British-born Churchill would have any type of definitive grasp on the rules of baseball, and frankly, after the second or third inning, pitching would probably pose a serious risk to his health. I don’t think Garfield has opposable thumbs, so any chance of him being able to even hold a baseball, much less wield the command necessary to compete against major league hitting, especially in the AL East, is very slim.
Come to think of it, this is kind of a dumb question. Josh Beckett is bad, but not this bad.