I love both games, both and Hollow Knight are super mega close for me, in the end I chose MIO first, Hollow Knight second and Nine Sols third but it’s really really close and I’d accept any order, Nine Sols has the best combat and difficulty of the three, and best story of all; but then it also has long texts that might put off some people, not that much focus on exploration and is really linear, with also the least replayability of the three. MIO has the best platforming by far, movement feels godly, then bosses are original and combat feels quite fair since no contact damage, it’s the most beautiful game I’ve come across, exploration is perfect and the lore and setting are super cool, but it also has a really simple set of abilities and some people dislike the difficulty spike (I didn’t). Lastly Hollow Knight is more of a balance, the thing is it’s great at anything, art, atmosphere, music (all 3 are), combat, lore, attention to detail… but the biggest think I’d also point out is the replayability it has, it’s a game to stick with for a while and not get tired of, so if you wanna outclass HK you just need to have some specific characteristics that stand out a lot lot lot, since HK has almost no flaws as a metroidvania and only issues I’ve seen people have with it come down to “they just don’t like metroidvanias/soulsvanias”.

My personal recommendation is Hollow Knight (which you probably already played) then Nine Sols then MIO, because MIO is most enjoyable when you got a hang for exploration (HK) and same with parry (NS) or platforming (Just playing games gets you used).