Which villain would be the closest to achieving Ultra Instinct?

[Hugo Boss]

Anyways, while I see the point of Kid Buu being the closest to getting UI, I don't really see him getting it, he fights with instinct but in a rather chaotic way, while UI seems to be more, controlled martial arts thing.

That’s because you are focusing on how Angels wield Ultra Instinct, but for Goku in his true version his heart is in a total drive for battle. Kid Boo is one of a few characters that match Goku in this aspect. His heart is totally focused on seeking combat.

[ankokudaishogun]

Tienshinhan.

[Magnificent Ponta]

A Saibaiman whose seed has been planted in the Angel Realm.

[Mr Baggins]

Moro technically achieved it already, though it was short lived and he couldn't control it for long.

Otherwise, I'm skeptical that any DB villain could get it authentically. Kid Boo's lust for chaos would probably stop him from reaching any zen-like state of mind, and that's basically his whole personality.

That’s because you are focusing on how Angels wield Ultra Instinct, but for Goku in his true version his heart is in a total drive for battle.

Wielding it like the Angels was the first stepping stone for Goku, though. Had he not trained under Whis and then Merus, he'd never learn to use it at will.