Simple clutch statistic:
To measure this statistic we will be looking at what are commonly considered to be clutch games. We will look at the permille of 'clutch games' to determine the clutch of the player. For example, if I win 16 'clutch games' and lose 10 'clutch games' my score will be 615
A clutch game is defined as a game with a comeback opportunity, an overtime opportunity, or a big game:
A Comeback Opportunity is defined as where the quarterback has possession in a one game where they are losing by fewer than 7

in the 4th quarter. (I.e. the Chargers loss to the Broncos wasn't considered a comeback opportunity because after the game became a 7 point game, the chargers never regained possession)
An Overtime Opportunity is defined as a game where the QB had possession in OT.
A Big Game is simply defined as a playoff game, where super bowl games count for double. Obviously here the argument is that clutch players are expected to perform on the big stage.
Assuming we use this as the definition of a clutch game, here is a list of all active players (minimum 15 clutch games) and a few notable HOFers (Unfortunately I couldn't find any data for Joe Montana on comeback opportunities; as such he could not be included in the list.).
For players who had a freak year that skewed their stats, I ignored the year (with their stats including that year posted)
This doesn't include the 2013 season, so some numbers (especially younger quarterbacks) will be off.
(Caps=retired)
JOE MONTANA 632
Tom Brady: 627
Eli Manning: 551
Matt Ryan: 516 (571 including the 2012 season)
Peyton Manning: 496
JOHN ELWAY: 492
DAN MARINO: 492
Jay Cutler: 486
Ben Roethlisberger: 478 (531 including 2004 season)
Russel Wilson: 462
Joe Flacco: 455 (511 including super bowl run)
TROY AIKMAN: 449
STEVE YOUNG: 448
Drew Brees: 442
Alex Smith: 419
BRETT FAVRE: 403
KURT WARNER: 400
Tony Romo: 400
Matt Stafford: 375
Matt Schaub: 361
Aaron Rodgers: 349
Andy Dalton: 333
Philip Rivers: 328
Carson Palmer: 317
Some notes (either stuff I found interesting or clarifications):
This attempts to measure clutch, and is not an overall measure of a players performance.
If you ignore Eli Manning’s 2 super bowl seasons, he plummets to a 351. He's either a god (833 average in those 2 years) or slightly below average. Eli Manning, Tom Brady and Joe Montana are the only 3 players to have recorded above 800 on more than one occasion.
I could only find data for Joe Montana for the 81, 83, 84, 85, 87, 89, 90, and 94 seasons. As a result his number is not a fully accurate representation of his entire career. Either way, I think its undisputed that Brady and Montana are the two best all time clutch players.
Tom Brady had the best single season, which was 2003 earning a score of 917. This was not an anomaly, has he has been able to post 875 the following year and 800 in 2010.
Joe Montana's best season was 1984, where he posted 909. He and Brady have both posted above 800 on 3 separate occasions.
Ben Roethlisberger scored 889 in his rookie season despite not winning the super bowl. He posted 636 and 769 in the years he won the super bowl. He, Brady and Peyton are the only ones who posted above 700 in a year where they did not win the super bowl.
Eli Manning participated in 18 clutch games in 2011 no other QB has participated in more than 14.
2 QBs technically scored better then Joe Cool on the all-time rankings, however I didn't include them due to lack of games (12 and 11 respectively). Tim Tebow (667) and Andrew Luck (636). With that in my mind, maybe Tim Tebow deserves a contract, lol
To my surprise, Peyton Manning is actually a clutch player, as he is 5th all-time in clutch games. He is also one of 3 players (Brady and Montana being the others) to post 3 or more seasons above a 667.
Aaron Rodgers is the prototype example of a super star who lacks clutch. Only on 5 occasions has he leaded a 4th quarter comeback, despite having 27 opportunities to do so.
Tony Romo is underrated. He has 18 'comeback' wins and a 4th quarter passer rating of 100.7, and a career passer rating of 95.6, which is the 5th in all time rankings. Given all the hate he gets (I'm looking at you Obi), he is one of the most underrated QBs of all time and second to Kurt Warner as the best undrafted QB of all time.
The above data took me almost 6 hours to gather and sort (you can thank adderall).
With the above data in mind, I'd like Montana to be moved to the #2 spot on my list. I'll admit to Obi that I vastly underrated him.
1. Tom Brady
2. Joe Montana
3. Peyton Manning
4. Jon Elway
5. Kurt Warner
6. Aaron Rodgers
7. Drew Brees
8. Dan Marino