For me, it was Drew Brees' perfect passer rating of 158.3 versus the New England Patriots in prime time in our 2009 season; I believe it was the 13th game, where we improved to 13-0, but my mind is fuzzy if that was the case.
I had never before seen Bilicheck and Brady with a total look of defeat on their faces, utter shock, to the point that he pulled his top starters with over 5 minutes left in the game, conceding defeat with a bewilderment on their face.
Payton was at the top of his offensive evolution and self scouting... what started in week one as Drew handing off to the HB in this particular formation alignment and personnel package, evolved to him faking the hand-off, pivoting to his right, and firing it a WR in the flat for a WR screen. A few weeks later, Drew would fake hand-off, pivot right, pump fake, spin 180 degrees and fire it to a WR in the left flat.
By the time we played the Patriots, Drew faked the hand-off, pivoted right and faked the throw, pivoted 180 and faked throw to the left flat, then squared up and hit Dave Thomas TE running a seam route unguarded down the middle of the field.
I can't remember watching a play evolve like that from week to week since Payton's forced leave of absence.
I also remember the Saints plan to basically shut down Randy Moss and force Brady to dump it to Welker and rally up for the tackle... they eliminated the Patriots biggest threat and did something very much Bilicheck-esque.
But also great about that game was that the Patriots were the measuring stick - the standard for greatness, and to dominate them in the fashion they did in primetime-- the only game close, for me, was when Payton's Saints forced Bill Parcells, as Cowboy HC, to simply concede defeat and start taking a knee with over two minutes left.
The Lions game in week 1 told me Drew and the offense would have an MVP pick your poison offense, the Bills game in week 2? told me they could find ways to win without much from the offense - a complete game with Special Teams, the Jets game in week 3? told me they could win with defense; the Miami comeback I think maybe in week 4 or 5 of that season told me this was a Super Bowl team... the Redskins game with Meachem's great play told me it was destiny, and the Patriots game removed any lingering doubt.