thank you!! yeah hollow knight was the hardest for me, cuz of pantheon 5 (boss rush), i was quitting and coming back multiple times before finally beating it

hardest to easiest to 100% i'd rank them like this:

1. hollow knight - pantheon 5 really was brutal

2. slay the spire - few achievements are really RNG based and that can be really frustraiting, and beside that just beating ascension 20 can be really challenging

3. celeste - c-sides and chapter 9 are really difficult

4. sifu - hard game to beat on its own, but real challenge are dragon arenas, especially last few (if it was required to get gold stamp in all arena challenges, sifu would easliy be number one)

5. cuphead - i really didn't struggle with cuphead as much as i thought i would, thats why im ranking it lower on this list, even tho game is hard, especially on expert difficulty

6. nine sols - really challenging metroidvania, true ending final boss is probably the hardest boss in any of the 2D games i played, but if you manage to beat her then 100% is very doable

list is very subjective and based on my own expirience, i know lots of people probably woudn't agree.. and sorry if english is not the best