I might even go as far as to say that the phrase "actually made the biggest difference" is logically incoherent.
What does that mean? That there was the biggest difference between what the player actually produced and what a replacement player would have accomplished? That makes sense, but then you are reverting back to "would." The player whose team got the best results? Obviously not what the MVP is for. The player with the best stats? Unless you can identify how those stats impact winning, that doesn't answer the question of who is the most valuable player, though it comes closer to the "best" player. Though it's probably true that there is no such thing as best player beyond most valuable player anyway, so it doesn't help much anyway.
Valuable is valuable is valuable. The question to ask if you want to compare 2 players is: which one do I want on my team? I think the better argument against Peyton Manning being MVP is that an injured player is not valuable, rather than that a player who didn't play is not valuable.
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2002 Link, 2003 Cloud, 2004 Link, 2005 Link, 2006 Link, 2008 Link, 2010 Link, 2011 Cloud.
Link 6, Cloud 2