Hmmm, two characters I adore... I'm going with Geo however
Short Story: This is tricky but Geo wins out logistically
Long Story?
In-character he's so much smarter than Kirby to the point it isn't even funny, at least from a combative and strategic perspective which gives him a major advantage here given their speeds are comparable.
@Reppu Kirby's hax may be comparable to Geo's overall, but in a fight with eachother most of it is useless while Geo's is still viable. Kirby has a wide variety of attacks... almost all of which would be useless here.
To explain why very nearly all of Kirby's powers are useless.
-Both have flight, making the advantage null, and Geo has much better and more frequent teleportation
-Both have lower-end Regenerationn, though Kirby's is more toonforceish going by the anime. However Geo's haxy/broken attacks won't leave Kirby with the ability to regenerate
-Geo can more than likely get out of Kirby's stomach dimension. In Mega Man Battle Network Cyber-Cores are just that and Geo can get out of them just fine, so there's no reason to say different for Kirby's stomach. And that is assuming Kirby can swallow Geo considering Geo is much larger and Kirby can't suck someone who isn't made of atoms to my knowledge.
-Elemental, Martial and Weapon-based attacks would accomplish nothing, aside from Spark/Plasma, as Geo is made of intangible waves
-While Spark shields would kill Geo, Kirby would not resort to that (he usually goes for physical attacks instead of energy based attacks, few of which would affect Geo anyway). His other shields are useless since Ice and Needles won't hurt Geo and will just make Kirby a sitting duck.
-Dura augmentation is useless and/or pointless since Geo's more haxed attacks ignore durability and the ones that don't won't do crap to Kirby
-Telekinesis is useless since Geo is both invisible, has no matter, and can teleport from it.
-Light manipulation is kinda pointless since Geo has dealt with light users IIRC
-Wouldn't Geo's noise level (And by extension power) increase with massive sound?
-Geo has vastly better teleportation
-I don't think Afterimages mean much, but Kung Foo Kid uses them.
-SF Mega Man is not organic and therefore cannot be poisoned
-Acid burns physical materials so Geo will again, not be harmed by it.
-Transmutation requires atoms. Guess who has none? Useless again.
-Is there any point in shapeshifting?
-Both have animal mimicry (though Geo's is more subtle)
-I don't know if magic will do anything, but not sure.
-Both have attack reflection (Geo can via equipped progams)
-Size manipulation may make Kirby a small and hard to hit target, but it also means he forgoes using any abilities that could harm Geo, meaning it only allows Geo more time.
-Ghost Kirby is not intangible (Replayed Squeak Squad to confirm), it's too rare for it to be likely to come into play, and EXE can resist possession so I don't see any reason why Geo can't either. Also Gemini Spark once mind-contolled Electopia and Geo wasn't affected, while even his friends were.
-Both can copy abilities, so it's a moot point.
-Time beam cannot hurt Geo, it's rare, and Kirby cannot use other abilities in time beam IIRC. Geo also has a possible time manipulation himself and can use abilities that would kill Kirby during time stop
-Kirby cannot ressurect himself.
Why Geo's abilities would work
-Kirby has no resistance to black holes, and cannot escape one that is spawned inside him or on his face.
-Radiation destroys cells in real life and has nothing to do with conventional durability. So while Red Gaia Eraser would not kill Kirby outright, the radiation could end up doing that.
-Turning atoms into EM Waves then scattering them is a very potent tool, one that could very easily kill Kirby and there's nothing Kirby can do to defend himself from it except for possibly Mirror
-Mirrors Edge is something that sucks soul and life energy, so again, Kirby has no defense against it.
-Geo might be able to BFR Kirby into a noise wave, seeing as Heartless did so with King.
Finally worth noting is that Geo is far less reluctant to use his trump cards in battle