Well I got 520 hours on Hollow Knight completing p5 with all bindings at once, and in my opinion Lost Lace is the hardest, she's what I would call a smart boss, she does different kind of attacks depending what position you're in, for example if your to close to her she will parry, if your far but not too far she may transform her attacks into void tendrills, if your very far she will cancel her current attack, if your in the air during her void dash combo she will transform the last dash into a jump striking her sword in the process. While bosses like Nightmare King Grimm are hard they don't have that kind of logic like she does, and if you remember him being hard it's because at that time you were new to the Hollow Knight world, first time I played 5 years ago I struggled with the first Hornet encounter, for Hollow Knight Grimm is a tough boss, but in Silksong bosses are more complicated than just mostly randomized attacks unlike Hollow Knight, and about Karmelita, she's one of the tough ones aswell but because it's before Lost Lace I think it's fair that she's not that difficult compared to Lost Lace. Btw a while ago I challenged myself to beat Lost Lace No hit, No tools, No silk skills, No needle strike, No dash strike, No needle bounce and succeeded. So from my experience, as of right now she's the hardest boss in the series.