He's not the best player on the Knicks. Using your logic, Zach Randolph was the best player on the Knicks, in the 2007-2008 season. He led the Knicks in Double Doubles that year.
David Lee is having all of these "double doubles", because nobody else on the Knick front court can rebound.
Who's going to compete with Lee for rebounds?
Jared Jeffries? Quentin Richardson? Danilo Gallinari?
David Lee has no moves. All he has is a lay up and dunk, off a pick and roll. His jump shot is much improved, but even his jump shot can be stopped, with defense.
David Lee is good, but calm down. He never has been and never will be a franchise player.
Al Harrington isn't a true franchise player either, but that's the role he's playing with the Knicks this year.