The moment Frog opens the Magic Cave. Sure, you had to give him a pretty big push. But from that point on, he is nothing short of a true hero.
This doesn't really do the scene justice. After recovering and repairing the Masamune, you take it to the cave that Magus' troops use to move around. He stops and listens, inspecting the blank cliff face, and has a flashback to himself and Cyrus. Then, he turns, takes the Masamune, and his theme begins to blare. A sudden pulse of light surrounds Frog, before shooting into the sky, and then he spins around and slices the mountain in half.
Marle gets one in a sidequest. Her father is about to be convicted unjustly by the Evil Chancellor. She's locked out of the courtroom, despite having found evidence to prove his innocence. What does she do? Breaks through the stained glass window into the courtroom, bails her father out, and kicks the shit out of the so-called Chancellor.
The whole series of events of bringing back Crono from the dead, storming the Black Omen, and fighting Queen Zeal and Lavos in the final battle to decide the fate of the future makes for one hell of a climax.
If Magus is in your party during the second Zeal fight, his battle theme overrides the one that should normally play, making the showdown all the more awesome especially since he's her son and she's too far gone to recognize him, as well as the lines he says.
To be fair, Schala didn't recognize him either. The only person who recognized him as Janus was Gaspar.
