Very subjective question Sleepy. Really depends on the era.
Those from the 1940's might argue Sammy Baugh or Sid Luckman. In the 1950's it could be Otto Graham. In the 1960's - when I started watching - it could be Johnny Unitas. Then going into the more modern era it could be Joe Montana, who I regard as the greatest clutch quarterback.
I couldn't argue with your choice of Peyton Manning. Certainly a valid pick. I will say Peyton was lucky to play for great teams with great skill position talent around him and for many years he had the luxury of playing in a temperature controlled climate at Indianapolis. Many other great quarterbacks didn't have nearly that many lucky pluses.
I would give Peyton the highest marks in intelligence, getting the ball out fast, consistent high production and durability. I would downgrade him in the argument of greatest ever for his poor track record in big games.
Certainly an interesting topic.