I can't think of anyone in the current game I'd consider an enemy and a favorite. I take no solace in getting torched by anyone on the Yankees, because it's happened too frequently, I can't name most of the guys on the Rays, the Jays hitters are young and likely to be a royal pain for years to come, and the Red Sox see the Yankees prior to this season.
While retired, strange as it is I have to go with Mariano Rivera. He always seemed like one of the classy players, had the respect of his peers, was an absolute master at what he did, and infuriating to go against because when you did 90 something percent of the time you were going to lose. But, in those rare instances where you did get the better of him it was so damn satisfying.