He's the best QB ever. You throw out Super Bowl wins with him, although I think he gets at least one more before he retires. He manages a game like no other. He makes everyone around him better.
Who else is in the discussion? I think you really have to break it down by era.
Its hard for most of us to talk about the guys from the late 50's / 60's: Unitas, Namath, Starr, Otto Graham, Y.A. Tittle. We werent alive to see them play, but those guys deserve a spot in the discussion. What about the guys from the 70's: Staubach, Bradshaw, Griese, Stabler?
I think the only competition Manning has for G.O.A.T comes from the QB's who played in 80's/90's. Montana, Elway, Marino.
The only ones from this era who deserve being in the talk besides Peyton Manning is Favre and Brady.