For me it's between Duncan and Manning. Both could possibly, when their careers are over, be considered the best to ever play their positions. If I had to choose one I guess it'd be Duncan. Not only does he have the rings, but he's both the anchor offensively and defensively for his team.
For me: Duncan Manning has a lot of records and stuff, but his game in big games, its not good enough, he always choked (until he won the superbowl) Duncan is big in playoffs games and finals.
It absolutely PAINS me to say it, but the only reasonable answer is Duncan. Stayed with the team that drafted him (eliminates Shaq) and has multiple championships and MVPs (I believe).
LeBron story is still being written... I'd voted Duncan stayed with the same team and won 4 rings there. But after thinkinga about it I would vote Peyton. He's a top 5 qb by most and there was alot of folks who wanted Ryan Leaf at the time. Most of those picks were no brainers at #1 but I do recall alot of debate among Leaf/Manning. If they botch that pick are they still in Indy?