I'd say that the openings to Mario Power Tennis and Toadstool Tour makes Wario and Waluigi the funniest of these choices. They also have their funny moments in the Strikers series (Mainly Waluigi no one can resist the Crotch Chop, also "Want the ball? HERE!" is a great one too). Luigi I think takes 3rd here because of Luigi's Mansion, the opening to Mario Golf N64, and some of the cutscenes after winning Mario Power Tennis tournaments (When he sees Shy Guy's real face)
Honorable mention to A LOT of the characters in the Mario RPG games.