The Raven 2 posted...
Apparently Bacon has forgotten about the appearances of L-Block, Missingno, and Master Hand. Barkley/Jordan would be more legit than all three of those
You know, I actually was considering Master Hand/Tabuu as one of my nominations, though I decided not to do it. Of course, a lot of the planned backstory in the Subspace Emissary got cut, so Master Hand's appearance in there amounts to a mere cameo.
The idea of a character whose only previous appearance was in the Villains Contest coming back in the "Protagonist" role in a rivalry contest seems pretty awesome, though. And in that backstory, Master Hand was attempting to save Subspace from Tabuu, only Tabuu won and Master Hand became his puppet. So that would totally work. And Master Hand still did appear in the game, even though the information that determines why this is a valid rivalry is from supplementary materials.
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Nominate The Kid vs. The Guy (I Wanna Be The Guy) for the Greatest Gaming Rivalry Contest!
Also, nom Luigi/King Boo, not Luigi/Waluigi.