system
Given the following post, respond as if you're commenting on Reddit:
human
[DBS] So now that we know that Ultra Instinct isn't limited to Saiyans, who would be the most monstrous with it?

Yamcha? Tien? Roshi? Krillin? Maybe guys like Pikkon, Piccolo and like Janemba or something?

Maybe some GT guys? 

Lets assume its possible to merge forms with ultra instinct, now whos the best? Would UI Goku be SSJ3, SSJ4, or SSJGSS?
gpt
The series already gives us the answer to that question: Whis.

He's a martial arts genius that even Beerus has trouble keeping up with.  He has the power to rewind and putz with time.  He likely has access to Hakai (since he serves a God of Destruction, it'd be handy for him to perform the same.)  And, worse yet, he's already got Ultra Instinct working for him.  In fact, he's mastered it to the point that he can keep it active 24/7 with no impact on his stamina (like how Goku and Vegeta can go SSJ1 all day without tiring.)  About the only thing keeping him from being "monstrous" with it is the fact that he's a pretty easy-going guy and doesn't much care for going on a rampage (he'd rather have some tasty parfait instead.)

If we're talking about figures who aren't already among the gods, then we also have the best answer coming from the series:  Frieza.  The thing about Frieza's character is that he's evil.  He wakes up evil, brushes his teeth evil, kills a minion for incompetence or just for giggles evil, eats dinner evil, and goes to sleep evil.  He does everything in just about as evil a way as he can.  And, it'd be one thing if he was someone like Buu or the Androids before Cell, where he was designed for evil and nothing else.  But, even those characters whose original defining purpose was solely for evil eventually turned to good at some point.  Not Frieza, though.  He's gone to Hell and back multiple times, and yet, knowing full well what kind of afterlife awaits him if he keeps making those types of choices, *he still chooses to be an evil bastard.*  Now imagine that kind of person being basically invincible.