It's the sum of its parts, there wasn't really one 20 second sequence that stood out to me, it was getting on the train, fighting bad guys on top of it, hanging from the sides, hopping down and having firefights and fistfights in the tram cars, then seeing the train come into that beautiful lakeside scenery, take out the helicopter with that insane setpiece moment right before you get into the tunnel, then into a snowy environment and taking out a chopper with an anti-air military vehicle, all the while moving a brisk pace? Just plain badass.
All of it just added up into an incredible sequence.