The generic "Great Guns!" didn't cut it. On the Superfriends, he used to say "Great galaxies!", I believe, which is, eh, okay. Once the blue guys appeared on the scene, he started with the "Great Guardians!", which is disturbingly obsequious, even for Hal.
I'm not sure whether what he came up with is a battle cry or signature exclamation or something in between, but finally he hit upon something --
"Spoon"? "Supercalifragilisticexpialidocious"? "Excelsior"? Forget them all. "UNMNGA!" is the only signature exclamation or battle cry you'll ever need.
"UNMNGA!" is what H.E.A.T. shouts when they storm the DC offices (or my house). "UNMNGA!" is what Ron Marz grunts when he dodges snipers. "UNMNGA!" is what stewardesses sigh in the wee hours as they stagger out of Hal's apartment building.
There are few situations for which "UNMNGA!" is not the ultimate expression. Now, if only I could pronounce it! Perhaps I need to get hit on the head more often...