My thoughts:
Terry Bradshaw 1st Superbowl appearance QB Rating 108.0
Tom Brady 1st Superbowl appearance QB Rating 86.2
Joe Montana 1st Superbowl appearance QB Rating 100.0
Terry Bradshaw 2nd Superbowl appearance QB Rating 122.5
Tom Brady 2nd Superbowl appearance QB Rating 100.5
Joe Montana 2nd Superbowl appearance QB Rating 127.2
Terry Bradshaw 3rd Superbowl appearance QB Rating 119.2
Tom Brady 3rd Superbowl appearance QB Rating 110.2
Joe Montana 3rd Superbowl appearance QB Rating 115.2
Terry Bradshaw 4th Superbowl appearance QB Rating 101.9
Tom Brady 4th Superbowl appearance QB Rating 82.5
Joe Montana 4th Superbowl appearance QB Rating 147.6
Who’s the best of those three in Big Games?
Montana is #1. Bradshaw is #2.
Those are compelling stats, and coming back at you over them feels a little homerish for me. Bradshaw and Montana have four rings each. Period. Until TB gets his fourth, those two guys are in a class by themselves. Please remember I said that as you read what I have to say below.
First of all, I think the question posed by the OP wasn't to identify the "best...in big games" (as you have it) but "clutch under pressure."
I think the analysis of Bradshaw, Brady and Montana yields different answers and shows that simply looking at Pass Ratings isn't going to answer the question that is on the table and not the one that you have interpolated here.
In SB IX, Bradshaw did get a high Pass Rating, but threw for just 96 yards in what was primarily a ground attack led by MVP Franco Harris' 158 rushing yards.
IN SB X, Bradshaw threw for 200+ yards, but the game was remembered for MVP Lynn Swann's two memorable catches. This is not to take anything away from Terry, but simply to say that this win was more about Swann than Bradshaw.
In SB XIII, MVP Bradshaw was indeed "the show." No doubt, no question.
Bradshaw was also MVP in SB XIV, but he did marvel afterwards at how he received the honor after throwing three INT's.
So, I think you can say that Bradshaw did what he had to do to win four big games. That is awesome. That is historic and he's in a class by himself, but I don't think it's a "dis" on him to say that these were not all "clutch" performances. In fact, I doubt he would say that himself.
In SB XVI, Montana was named MVP of a game that was characterized by turnovers. He received the honor because he took command of the game at two critical junctures. It was definitely "clutch" even if it wasn't pretty.
SB XIX was a blowout. Montana was great, but "clutch" doesn't describe it.
SB XXIII was one of the greatest SB games of all times in which Montana led his team back from behind in the closing moments of the game. This was indeed a great, clutch performance, one of the greatest of all times in a big game.
SB XXIV was a 45 point blowout, led by MVP Montana. Super performance, but "clutch" has never been used to describe it.
So, two of Montana's SB wins were true clutch performances; two were laughers.
Tom Brady's performance at the end of SB XXVI was one of the greatest clutch performances of all times. As John Madden put it, "What Tom Brady just did gives me goose bumps." It is disrespectful of Brady and of the game to say that this was not clutch.
At the end of SB XXXVIII, aided by an errant Carolina kick, Brady again led his team to a last minute, game winning FG. That too was a great clutch moment in SB history.
SB XXXIX was characterized more by Philly's ineptness in the fourth quarter than it was by history-making Patriot play. But, like Terry Bradshaw before him, Tom Brady found a way to win this game, sealed by a Rodney Harrison INT in the closing moments.
As for SB XLIII, "winners tell stories, losers make excuses." Brady led his team to a four point lead with three minutes to go. The D couldn't hold the lead and Brady was out of miracles with no timeouts from his own 13 yard line with a minute or so to go. There are those on this board who will say that the Giants were lucky and that Brady would be remembered for the comeback if Tyree hadn't trapped the ball on his helmet. To that I repeat one of Bill Belichick's favorite sayings: "The winning team is the one that finds a way to make a play when it has to make one." The Giants did; we didn't. End of story.