As a Red Sox fan, I wish I was writing that he was currently playing for them and was doing very well. Despite this, I think that it is fair to say that Jason Bay right now is the best player in baseball. He is doing so many things right.
Mike Trout has a higher WAR, better average, has already won an MVP award, is one of the best defensive players in baseball, and is an all around talent. He can hit for power, yet steal bases too. Not to mention Angel Stadium is a very pitcher friendly ballpark. Despite that, he's hit 40 home runs this year alone. Imagine what he could do in a hitter friendly ballpark like Oriole Park or Coors Field.
I don't even think he is in the top half of all Major League ballplayers. He was a good ballplayer once upon a time but in recent years, he has played well below expectations. He had one of the worst batting averages last year and his power numbers have significantly decreased since the mid-2000s. In my opinion, the best Major Leaguer is Miguel Cabrera.
I think that Jason Bay is a very talented baseball player. He is probably in the top five in the league right now. But to say that he is currently the very best player in all the MLB is going a bit too far, in my book. He is still good though.
I think it is incredibly difficult to identify one player as the "best" in any given sport, particularly baseball. Baseball has so many aspects; batting, running the bases, fielding, etc. While Jason Bay does so many things right, I don't think he is the clear cut best player in the MLB. I think the only way to say he is clearly the best is for a large group of people to agree, but I don't see that happening in this case.
Jason Bay is a great player but he is not the best in the game right now. The best baseball player in the MLB is Miguel Cabrerra. What he did last year was absolutely ridiculous and I expect the same in 201. Cabrerra is a better hitter and fielder in my opinion.