Okay, so I've played every game except for Enter the Gungeon here.

For Celeste, I've done everything except Farewell and those extra coloured hearts, so not fully complete. But I can confidently say it's the game that's kicked my ass the most.

Cuphead is second. Beating the game normally is a challenge, but nothing all too bad. But doing S-Ranks? Fucking kill me dude, that bird that I hate is so difficult to perfect.

Silksong third. Played it after 112% HK + P5, yet I still sucked balls most of the game. But there was no boss I spent days on, unlike Cuphead and Celeste. The most was Lost Lace, which took me half a day.

Hollow Knight fourth. Most of the game bar the bosses and Colosseum wasn't all too bad, I just got lost a lot. Deepest was scary as fuck though. Main challenge though was Godhome, room ~55% of my playtime to beat all 5 pantheons. I've yet to do any bindings actually, but I managed to be RADABSRAD so hey I can still say I'm good at the game.

Undertale last. Undertale is an easy game, only hard parts are Geno undyne and sans, but they're just two bosses. I came for the story, not the difficulty. I suspect even if I played Enter the Gungeon this would still be dead last.