The first play that came to my mind was the Troy Brown / Antwan Harris blocked FG return for TD in the 2001 AFCCG.
For more recent fans its hard to understand where we as a fan base were then. We had not ever won anything, had suffered through many horrific years (1-15 anyone?), and were huge underdogs to the mighty Steelers with their storied history and multiple SB victories. NO ONE gave us a chance, and I think even us as Pats fans were hoping for the best in that game, but not expecting much.
When that play happened, there was a big mental shift. I believe it changed the score from 14-3 to 21-3, and with that, we realized that we could win that game, and the AFC, and so on --- a turning point in the history of the franchise.