When Pox kills the Master in the third game. The fight with Roboprez and Silhouette in the first game.
You usually feel rather satisfied after knocking down an entire city.
The Kojira Battle in the second game. You could easily be expecting some lame classic sort of boss battle with the beast emerging from the middle of the stage in the volcano, but no, Kojira heads down to the city and starts smashing it to pieces!
In 2, you get a gun that summons METEORS!
That's nothing. In Path of the Furon, you get a pistol that opens up a frikkin black hole, a gun that summons a man-eating plant, and a saucer weapon that creates tornadoes. Epic.
The mothership's destruction in the second game.
In the second game, after struggling to come up with a reason why the Cosmonauts should turn against the Blisk, Crypto finally comes up with the perfect one: