Okay, sorry for being late to the fun! Regardless, there's very clear things going on here that need to be addressed.
Mario's speed: OP states "Mario." And never really limits him to the main series woth of games, therefore, I should be allowed to use Mario Party 6, where Mario escapes a moderately-sized black hole, making him very clearly FTL in travel speed, MFTL+ in reaction speed.
Mario's reaction speed VS Kirby's: Mario's wins, and I'll explain in a few seconds. The game always refers to the galaxies in Mario being "Galaxies." Whether you wanna judge them just by visual size or not, that doesn't change the game saying their galaxies. And that's that. Hell, the Mario series scales things down PLENTY of times, Super Mario World being my most notable example. And in Galaxy 1, Polari even states that the Comet Observatory reflects galaxies, & the Power Stars extend their sight to other further galaxies. There are a total of 70 Galaxies in Super Mario Galaxy, and the nearest Galaxy to us is called the Small Magellanic Cloud, which is
210,000 lightyears away.(There's proof)
Using that as average distance, let's multiply it's distance by 70, going to the furthest Galaxy in Mario Galaxy. 210,000 x 70 = 14,700,000. 14,700,000 lightyears. Let's use the screen as, well, the most popular answer. Ten seconds.
14,700,000 / 10 = 1,470,000. So, every second, Mario was traveling 1,470,000 lightyears, reacting perfectly fine. I.e. him moving, or, adjusting aim, & preparing for landing. Heck, he could even stop his speed in an instant, which is how he doesn't harm himself or the planet.
@Arbitrary: The most notable feat you brought up is Kirby traveling 3 galaxies in less than a second. Let's say he traveled six, shall we?
Using the same numbers for fairness reasons, 210,000 x 6 = 1,260,000 lightyears every second.
Mario reaction speed: 1,470,000 lightyears per second.
Kirby reaction speed: 1,260,000 lightyears per second.
Who's faster? Mario, clearly. Nice try, Kirby.
Mario VS Kirby: My vote's going for Mario, and here's why.
So, let's start with AP & durability. Kirby's Multi-Solar System Level, approaching Galaxy Level, right? Mario's superior to a Galaxy Level. But, well, that's not all. Mario's fought enemies that put him too far above Kirby. Even the whole verse!
Dreamy Bowser: "Oh man, I have so much power, I can do whatever I want! This island, this world, all of it... MINE! And you guys are... COOKED!"
Culex: "I can see the past. I can see the future. I am matter. I am antimatter. I consume time, & I'll consume you!"
Chakron: "With the power of Earth & Space, I can do anything."
"I am above, & all around. I am all that is you."
"There is nothing that Space knows not."
Dreamy Bowser, Culex & Chakron are the reasons Mario should win this. Possibly even King Boo & Dark Bowser, who I'll get to afterwards.
Those are quotes from Dreamy Bowser, Culex & Chakron. The last three being seperate quotes from Chakron. Dreamy Bowser can destroy the world. Sounds planetary, until you find that Mario language states that a world is a Universe.
Luigi even states that the world your in in Super Mario Galaxy 2 is "the Universe." Super Dimentio backs up that claim,
& Miyamoto saying everything in Mario is just Mario, no "canon" or "non-canon," I think it's clear that with how frequently worlds are called Universes in Mario, it can be taken as fact.
So, Dreamy Bowser's Universal at least & has power to practically do anything, same with Chakron, who also clearly shows he's
Omnipresent & Omniscient at the least. Being able to have enough power to do anything sure sounds like Omnipotence, but I'll leave that term alone, since it always has been vague. Culex is Universal, possibly Multi-Universal, very clearly extremely powerful. King Boo created a dimension, Universal, & Dark Bowser destroying the world PLUS Bowser's power, at least Universal, Mario's just far too powerful.
So, my vote? Mario, with ease.