The Best Quarterback in History is usually The Quarterback of whatever team wins the Superbowl each year, or is in the most commercials. Currently Peyton Manning is in at least 73.3% of all commercials so he is the best quarterback right now.
The Quarterback position gets most of the credit when a team wins and gets most of the blame when a team loses. Their main responsibility is to take the snap and either handoff or throw to an open receiver. Even though this sounds extremely easy many struggle to do both, or in the case of Rex Grossman, either. In the early days of football guys like Sammy Baugh would also play defense and special teams, but now most pro quarterbacks are sissies who just like to look at themselves in the mirror.
The best quarterbacks win Superbowls. Joe Montana and Terry Bradshaw are the only quarterbacks with 4 Superbowl wins. Bradshaw is not really often considered one of the greatest due to his bat shit craziness and IQ of less than 4. Tom Brady and Troy Aikman have each won 3 Superbowls and Brady has been in Giselle(Which is way more impressive than winning a Superbowl). Joe Namath won 1 Superbowl and is considered one of the greatest even though he threw more interceptions than touchdowns and had a QB rating of less than 70. But when you look like this people forgive that.
Usually Quarterbacks are ranked among there peers by wins, titles, and stats. The top ten quarterbacks are usually listed something like this.
1. Joe Montana
2. Johnny Unitas.
3. John Elway
4. Peyton Manning
5. Tom Brady
6. Dan Marino
7. Terry Bradshaw
8. Fran Tarkenton
9. Roger Staubach
10. Brett Favre
Lists like this can be found all over the internet, while most the time it will be in a different order in doesn't matter at all. Everyone on any of these lists and is cooler than you and had a way more awesome life, so don't get to upset if Montana is ahead of Manning. I'm sure Manning will be just fine wherever he is at.