I think Chaos takes this one, as much as I love Discord. It's a really close battle, especially with speed equalized to remove Chaos's omnipresence, but stuff like Chaos's Type 8 Immortality, which Discord notably does not have, gives him the slightest of advantages to edge this out.
Seeing as how Chaos lacks abilities and coverage, and how Discord doesn't usually go for kills (making Type 8 essentially useless here), Discord power nulls before Chaos can do anything or can get anywhere near with that hefty AP advantage, which may also prove unreliable due to how tricky it is to hit Discord between his regen and intangibility. My vote is on Discord for now, but will wait for an answer for what Chaos leads with.
All these chaotic reality-warping matchups are making me want deltarune to release already so that Jevil joins along.
Chaos stomps. He has 4 whiskers, each with more magic than an entire palace worth of genies. That means his whiskers alone have over 4000 times as much power as a normal genie, to say nothing of the rest of his body. Honestly, solar system level is way too low for him. Discord meanwhile doesn't have enough strength on his own to match the combined power of Twilight, Celestia, cadence, and Luna, as Tirek was only able to match all alicorn Twilight after combining Discord's power with the power he had stolen from hundreds of ponies. Discord has less magic than four star level characters put together, while Chaos has the power of over 4000 solar system level beings. I don't care how much versatility Discord has, there's no way he can beat someone with so much more power than him. Not even Tirek with all the magic in Grogar's Bewitching Bell, all of Starswirl, Starlight, Discord, Cadence, Celestia, Luna, Flurry Heart, and Twilight's magic absorbed could be even one tenth as powerful as Chaos. It's simple math. Four Large Star characters vs 4000 Solar System level characters? Imprisoned Genie alone is listed at Solar System level, making him alone more powerful than Discord.
Oh. Well I am still going with Chaos because unlike Uncle Grandpa, he has true combat experience and is not as peaceful. He's also way, way, way more intelligent than UG and has likely been around far longer than Discord and likely has more experience than Discord. UG is a peaceful idiot with little to no combat experience while Chaos is a nigh omniescent being with millienia of combat experience.
Chaos was only in one episode, so it's hard to predict what his first move would be. However, his most used abilities seem to be invisibility and transmutation. He constantly turns invisible and turns people into animals in the episode. He always seems to use invisibility whenever he first appears, which could allow him the chance to strike first. Discord might have trouble power nulling him if he can't see him.
Then it’s a good thing Discord has Enhanced Senses and he can detect magical disturbances, he’ll definitely know where Chaos is. Shapeshifting also takes care of Transmutation. Seriously his powers are crazy, and can do anything Chaos can, plus more.