Without question, KG.
It always amuses me to read the game recaps this year and see the word "vintage" come up as often as it has when they describe KG's play. KG is who KG is. He doesn't tap into his youth every now and then when he posts great stats. News flash- he's still good. You're allowed to use "vintage" when Vince Carter explodes for 30 points or when Gilbert Arenas sinks in a game winning shot. But that's simply disrespect to KG when you throw that word around to describe his good nights.
Bill Simmons was so wrong about KG's knees. "When a big man's knees goes, you're done." He's back and he's still performing at a very high level. When the game is close in the end, it's always KG who you can depend on to deliver. The shot at Golden State, the clutch rebounds, the defense, the passion on the bench, the encouragements, the intense practices, the laying down on the floor yelling and waving his hands in the air after he takes a charge, he's the heart and soul of the Celtics.
Pierce's our lovable captain but KG is the unquestionable leader. The only reason Rondo is able to lead the team on the court is because KG allows him to.