For me it was the defense. Great 1st half vs the Chargers, great 1st half vs the Chiefs, and then a ridiculous full game vas the Rams. The Chiefs and the Rams have 2 of the top 12 scoring offenses ever, and to play 6 quarters of perfect defense against those 2 teams was incredible to watch. Scary to watch 2nd halves vs the Chiefs and Chargers, but that 1st half defense gave up 7+0+0 in the 3 playoffs games, a total of 7 points against some excellent offenses.
Then the last 2 scoring drives by the Pats in the SB. I kept on saying, "Brady, it's time to Brady them" all game, he finally did it in the TD drive that pretty much ended the game. The way that defense played, 7 points might as well had been 70 points. Best defensive performance I've ever seen. The Rams never got past the Pat's 27 yard line, NEVER got into the Red Zone, I think that only happened once before (Cowboys vs Dolphins in SB 6 I believe). Maybe the Ravens vs the Giants, the Giants only scored on a kick off return.
Regardless, I consider the Pats defensive performance the most dominating ever in a Super Bowl, because who they were playing, The Ravens were not facing a powerhouse offense in the Giants, The Cowboys were not facing a powerhouse offense in the Dolphins. The Pats were facing the 12th highest scoring offense EVER. 3 points, and zero trips into the red zone. Incredible.