Bowser: I dashdance back and forth, wavedash back and forth, or F-Smash the air. If it was a Star KO, then I do a U-Tilt right when their character turns into a star.
Mr. G&W: U-Tilt x2, turn around, U-Tilt x2. If I have any more time, I might taunt or use Judgement Hammer. Perhaps Oil Panic. Taunting repeatedly is fun, even in the middle of an intense battle. Oh, and 98% of the time, I never actually run with G&W; I do a quick dash and wait for him to stop, and then repeat. Useful mindgame, and it looks cool as heck.
Young Link: Boomerang, bomb, taunt. I might just spam D-Tilt or something for no apparent reason, as well.
Zelda: On the ground, Up-B downward. Taunting is nice, too.
Link: F-Tilts. I'm a CPU in disguise. <3
Luigi: Green Missile for great justice.
Donkey Kong: Charge the punch. If it's fully-charged, I'll taunt repeatedly, even in the middle of a match.
Jigglypuff: Dash for a step, then let go, then press forward again. When you time it right, it looks like Jigglypuff trips.
Everyone else, I can't think of anything interesting.