The argument for Marth was that the Falcion made him OP in the very first FE game for the Famicom (made him invincible to everything except dragons, which are weak against the Falcion anyway, and gave him a sizable healing factor). This was later discounted as later games, including the remake, make the sword more reasonable (mainly keeping the dragonbane and healing factor).
Ganondorf's sole weakness was heavily discussed a lot too, but I personally felt that most universes had holy equivalents to get around (Mario Star Power, Sonic's Chaos Emeralds, Samus' Light Missiles, etc).
Pokémon Trainer's ranking was based on what he could get. For example, there is a difference between Gen 1 only vs a team of 6 reality-bending Legendaries.
Rosalina...I don't even want to get into
those discussions. Let's just say the No Limits Fallacy was in full effect and then some.
Mewtwo was also heavily discussed as to whether inducing amnesia and otherwise incapacitating the opponent counted a victory (e.g. Erasing their memory and warping them to, say, Tennessee where they could live the remainder of their happy lives without ever seeing Mewtwo again, at which the latter will win by outliving them, or warping them into space where they essentially cannot move).
Pit and the space animals were discussed on whether they could use their vehicles or not, as the Great Sacred Treasure is a war machine and not much could survive being hit by the Blue Falcon or even reach the Arwing. One guess was to have the characters only use their vehicles against others with similar vehicles.
Nearly forgot about Shulk. His Monado and future sight is fantastic, but he is as squishy as the White Mage in game and his battles are meant to be fought in teams, never solo.
All this said, nearly everyone agreed that ROB was dead last. He doesn't exactly have a canon portrayal (SSE doesn't count) so it was assumed that he was...merely the peripheral, something even Olimar could beat.
