LOOL silver age superman , and maybe even modern or new 52 LOL stomps em

silver age superman could lift eternity , hold blackholes, tow 13 planets with an arm, sneeze solar systems away and WENT SO FAST HE SURPASSED time , space, reality AND IN THE END had to be halted by God himself lol

And this is why Super Man games, comics and movies all suck. Batman is much more interesting.

Kirby can beat them all since he can just steal their abilities (except for Superman, since he is invincible)

Kirby would suck so hard that the Marvel characters wont know whats coming

No one could be able to take down The Flash.

EDIT: unless Kirby surprise stealth-absorb The Flash, no one can truly defeat him. If that circumstance isn't met, Kirby is lost as well.

Toad could take the Hulk. I'm pretty sure that he could also take on Superman if he can ground him. Maybe hit him with a kryptonite turnip first.

What makes you think Kirby would loose to Iron Man? Kirby is ultra over powered. He can consume anything and gain the abilities of what he consumes. The only one I can think of that Kirby might have a hard time with is Rogue since she has similar abilities and can temporarly steal powers by touch. So unless Kriby can consume her before she touches him, he has probably got her with the exception of her touching him from the inside, but then they both gain steal each others powers and the world probably implodes lol. 

Not sure about invincible, he's still weak to cryptonite. Darn close though. I still think Kirby could potentially win lol. Eat superman, gain his powers, near invincability as well but doesn't gain the weakness to cryptonite lol. Kirby is a nasty little puff.

ScrewAttack determined that Kirby could defeat Majin Buu from DBZ. So he's basically Goku tier. I guess Superman could take him. If Kirby can't beat Superman I don't see how any other Nintendo character could.

A race between The Flash and Deoxys Speed Forme could be fun, although I'm sure Flash would win handily. For reference, Deoxys Speed Forme is 12.5% faster than Ninjask, a two-foot-long, 26.5 lb bug that can fly fast enough to be invisible to the naked eye. Actually, Ninjask would probably be the better choice, since unlike Deoxys it can learn Agility, a technique that allows it to triple its speed. Maybe that whole 'invisible to the naked eye' thing is supposed to be while using Agility, though.

PS if Kirby swallowed the Hulk and acquired his power he'd probably be 110% unstoppable. Unlike the Hulk, Kirby has bizarre reality-warping powers. Who knows how those would be affected by some Gamma Radiation and angry eyebrows.

He might gain the weakness to Kryptonite. But then he could just eject Superman's power and eat the Kryptonite. That would be a pretty handy form for dealing with Superman.

First off. Kirby can't beat anyone. Seriously, Elektra could probably beat him. Kirby can't suck up any particularly strong enemy that puts up a fight, and not every enemy gives him an ability. Even a guy like Poppy Bros can't be sucked up until rendered unconscious.

As for who could take out Marvel's more powerful characters... not many. Samus could take out guys like Spider-man, and maybe Iron man. Would lose to guys like Thor. Mewtwo's psychic powers make him formidable, but Marvel's psychics outclass him. It's also pretty unclear, if we go by games, what limits he or any other Pokemon have. They claim to be universe creators and such, but can be hurt by average Pokemon. I'm actually just going to throw out Pokemon altogether.

Shulk *spoilers* could theoretically take out the Marvel U as a whole. The Monado literally controls the fabric of the universe, so if its potential was unleashed Shulk could literally become god. He is conceivable as powerful as infinity gauntlet thanos, but he doesn't show that level of mastery in the games. There are marvel characters with similar abilities (scarlet witch) who are more adept at using them, even if not as powerful.

Unless we want to count Bayonetta, there isn't anyone in the Nintendo U I can think of that can physically compete with the Hulk, Sentry, Juggernaut, or Thor. Few characters that can really compete with lower powered heroes like Spider-man or Captain America.