Most of you are confusing 'best player' with 'MVP'. There is a very strong possibility that they can be the same person, but the definitions of each term are different. MVP means "who is the best player that is most important to his team". Essentially, make a list of the top players and take him out of his team's lineup and see which team becomes the worst.
In 1987, Andre Dawson won the MVP and was on the last place
Cubs. This stat line was not be the best in the NL at that time:
full voting record: http://www.baseballreference.com/awards/awards_1987.shtml#NLmvp
One could easily make an arguement for others players who had better overall numbers and played on much more successful teams. However, take Dawson out of the Cubs lineup and they get hurt the most. I think the same could be said for Derrek Lee this year. Everyone else on the Cubs has had a fairly up-and-down year and that team could not have survived at all without Lee in the lineup. St. Louis is still an very good team without Pujols, and Philly is almost serviceable without Abreu.
Lee is the clear winner at this point in the season.