Zack gets one in 4 when he prepares to take on Alpha-152 with the simple declaration of, "Sometimes a man's gotta do what a man's gotta do!"
Beating Alpha-152 on 4 with anybody is one for the player.
Ayane, Hayate and Hayabusa's endings in 4. Even better is that they're used as cutscenes in the Story Mode for Dimensions as part of a series of sequential events.
Genra has Hayate and Ayane on the ropes in Dimensions, what happens next?, Ryu throws a kunai at the bastard's head, then slices, & stabs him with the Dragon Sword, stunning the guy long enough for the former two to finish him off with a Kamehamehadoken. This, ladies and gentlemen, is the power of the Super Ninja.
Jann-Lee punching out a T-Rex in 4. (Even if Hitomi doesn't think so.)
Ryu following a recent trend by making his Story Mode entrance in 6 a Big Damn Hero moment when he effortlessly takes down three Phase-4 clones that had surrounded Kasumi.
Later on, he takes down a helicopter that had been firing machine gun rounds and missiles at him with one shot to allow Helena's own chopper to land.
The final battle against the resurrected Raidou. The fact that the player can choose any of Kasumi, Ayane, or Hayate to fight him makes it even better given It's Personal for all three.