Mwuahaha, Waluigi the winner. As of now, Waluigi has only appeared in spin-off games; there havn't been any games called "Waluigi World" or "The Adventure of the Great Waluigi". Although Nintendo don't generally name games like the latter anyway. However, I think that this should all change.
Waluigi should have hi9s own game, where you play as him, or he could have a cameo without being a title character. He could even be an NPC as the final boos on a game or something; or a normal boss, a mini-boss, a sub-boss...
He has, however, played a significant role in the spin-offs that he has appeared in. He was the final boss in Dance Dance Revolution (was that the name) in which he was the main baddy and the final boss. Also, in Mario Party 3, he was the only playable character in the game who had their own stage(as far as I remember, though Daisy had a duel stage) which meant that he wasn't playable in the story mode, although who gets Mario Party games for that.
However, he hasn't yet been named in Smash Bros Brawl, and we know he wasn't in the other 2 because they are already out. And this, in my view, should be rectified. Waluigi should be announced on Dojo tomorrow because while he hasn't yet had his own game, and Smash Bros isn't exactly a spin-off, IT IS LIKE 1 IN MY VIEW. What do spin-offs do, they get together a whole bunch of characters and make them do stuff. Whether it's race, play football or fight, that's what they are. Brawl is like that but from many series.
Too lazy to conclude.