Saturn, after this chapter, is firmly in the top 10 as far as I am concerned. Luffy literally tries to throw him into the sea when he realizes that his actions have no effect, and he just flies back still. And from the way Luffy acts, Acoc/Haki does not seem enough to damage him either. People like Kaidos whose battle IQs aren't even high enough to fucking try anything but continue to attack? Are pretty much doomed to fail against Saturn since just brute forcing him has thus far done nothing to him at all.
If his regneration is not shown to have a time or stamina limit really quick? Saturn having his insane regen plus eye attacks capable of harming Gear 5 Luffy plus a poison coating that is so strong it literally just erases shit out of existence is a broke combo for literally every other character to go up against.
He is basically a buffed version of Magellan that can summon his four similarly or stronger buddies whenever he feels like it.
Bro is HIM: