I said 'best player', but I actually meant 'best season'. Still, I think that a player who is not having the best season can be the most valuable player. I often think to myself, which player would I hate to lose the most for the season? It's often a very close call, but it's not always the player having the best season.
For example, I'm inclined to think that losing Papi or Pedroia this season could have impacted the team more than losing Mookie. Maybe not this year because Mookie has been otherworldly, but I think you get my point.