To begin with, there is no right answer, it's an opinion. There are 2 factors, stats and winning. Winning is the ultimate guage, but this is a team sport and the Qb is only 1 player and some of his success or lack of, depends on his supporting cast (players,coaches,owner,etc).
1. JOHNNY UNITAS-Unitas had the stats, when he retired, he held almost every passing record in the books, the game has changed and many of those records have been surpassed, which takes nothing away from what he did and when he did it. He also had the wins, with a 3-2 record in championship games, although he wasn't a full-time starter in one of the losses and one of the wins. The thing that will always seperate Unitas with the rest of the pack, was his place in the history of the NFL, he was the right person, at the right time, he was the Babe Ruth of the NFL, he changed the way the game was played and was the first Qb to do many of the things we now, take for granted, to say he was legendary, would be an understatement!
2. TOM BRADY- this may not be what Colts fans want to see..... I can't stand the patriots, but have never really had a problem with Brady , I always admired the guys who made it to the top the hard way, moreso than the guys who were born superstars. I guess in my mind, they deserve it more. They made it happen, with hard work, nothing was handed to them. He also has the stats, not quite on par with some of the rest, but he has some very impressive stats. Winning, he was one of the best , with a 3-2 record in championship games, when I say winning, being in a championship game is winning, in my book, if you win the game, ever better. The 1 thing that I think really helped Brady, was playing for one of the best coaches of his era, that was a major contributing factor.
3. (tie) JOE MONTANA- Montana had the least impressive stats of the group, but the most impressive winning factor, he rarely led then league in anything, but wins. His record in championship games was 4-0, although he had a very dominant team to support him, that can't be overlooked.
3. (tie) PEYTON MANNING- his stats are as good as anyones, dominating his era and will go down as one of the best of all time from that perspective, the winning wasn't, with a 1-1 record in championship games, the second lowest totals in the group. The 1 thing that I always noticed is how the Colts were always right there, just not quite good enough. Being active, there is a chance that could change , before he retires and could put him higher up on the list.
5. (tie) BRETT FARVE- the king of stats, who curently holds most of the career records for QB's. As far as winning, he was also 1-1 in championship games.
5. (tie) DAN MARINO-probably shouldn't be on the list, but I really feel that these 6 Qb's stand above and beyond all of the other Qb's in NFL history and only these 6, so I included him. The stats are right there with the best of the best, he was maybe one the best pure passers the league has ever seen. The winning didn't happen, he made only 1 appearance in a championship game and lost.
Summary: I placed a great deal of emphasis on winning, but tried to use all factors in coming up with the list. As I mentioned, football is a team game and any QB has to have a great supporting cast, how much success was due to the players teamates, coaches, etc? I also feel if you want to be considered as one of the best-ever, you have to find a way to make it happen and be the difference ,