Super Dimentio, but he's ugly so it doesn't matter.
Otherwise, depends on the character's mood. Bowser with Star Rod could technically beat anything by wishing it away, but Bowser is an idiot. Majora has a ton of undefined powers, but Majora is crazy so the odds of it using said powers are small. The final boss of Fire Emblem 10 has the same Ganondorf stipulation AND doesn't suck at killing things outside of that, but it's apathetic at best about giving an effort.
So yeah probably Giygas because he's literally too stupid to screw around.
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SuperNiceDog? More like SuperChillGuy. Who is better than me at brackets by a mile.