In "The Sphinx," Egon, who had been feeling pretty useless due to his age for most of the episode, becomes the Big Damn Hero because of his age and experience. To elaborate, he confronts the titular ghost (all while walking with a cane) and correctly answers the riddle.
"Back In The Saddle": Both teams coming together to defeat one of the biggest creatures in the history of the franchise.
When the government agents arrest the Ghostbusters in "Grease," Garrett promises to sign a full confession if they'll only take his cuffs off. When they look doubtful, he snivels, "What am I gonna do? Run away?" When they do so, Garrett promptly kicks the two agents' asses, while aboard a plane. The best part has got to be the rest of the team looking at each other while he's pleading, like they know exactly what he's going to do, and they're right.