Typically I'd just reference the stats, which are in Manning's favor. He holds the record for touchdown receptions in a single season, has a higher passer rating than Brady (although I don't ascribe that much merit to the rating system), every year is pretty much an automatic 4,000 yard season (his rookie season and last year being the only time he came up a little short) and the last 3 years his completion percentage was close to 70 percent which trumps Brady and pretty much everyone else in the league right now. But there is so much more to Peyton Manning than the stats.
He has every intangible you could possibly think of: leadership, awareness, field vision, work ethic. He's not a very good rushing quarterback, but he has so much pocket presence and general awareness that he can avoid the rush and shoot of a pass to the open receiver better than any quarterback I can think of. Their pass blocking, therefore, doesn't even have to be that good because his awareness and skill level are so high it negates a subpar offensive line. Rush blocking is a different story, but he can also negate a weak running game (as you've seen this season). He also compensates, to an extent, for a bad defense. Of course this is purely conjecture, but I cannot believe any other team in the league with a defense like Indy's would threaten to go undefeated 2 years in a row in the modern NFL. They are 8-0 right now, having gone on the road and beaten 2 very good teams in New England and Denver, despite having THE WORST RUN DEFENSE IN THE NFL. This is really, really impressive if you think about it.
And he is the best simply because he is the most intelligent player in the game. The way he reads defenses and calls audibles at the line, the way he knows when to take a sack, the way he knows how to manage the clock. He knows exactly what to do and makes virtually zero mental mistakes. Just go back and watch the tape from that Monday Night comeback against Tampa Bay. If you need a score to save your life, Peyton Manning is the guy you want at quarterback.
Which is why I think it's unfair that due to a somewhat below average defense, he has never won a championship. Because if you have to play all the top-tier teams in the league consecutively, the odds are your defensive weaknesses will be uncovered at some point and exploited. He can only carry them so far.