I've come to this conclusion:
Having played lots of difficult hacks, and the following three, I think the hardest hacks ever are rated in this order:
1 - Wall jump abuse
2 - Glitch abuse 2
3 - Living on the edge
The non-walljump bits in walljump abuse are actually what I found hardest. Walljump abuse has the hard parts in glitch abuse 2 and makes it harder, like disco shell hopping and shell juggling, to keeping invincibility frames and keyjumping. Glitch abuse 2 has those devilishly long shell jumps though, which would put it abuse wja, so it's a pretty close second. Living on the edge is practically a condensed version of both, with slightly easier parts. The things I would consider difficult are the block duplication at the beginning, the note block duplication, and maybe keeping running speed in parts. Wja also has more frame perfect jumps needed, with variety of such things.
I can't vouch for pitiless, since there are no video's of it, nor have I gotten very far in it. It could be classed in a completely different category, due to puzzle aspects and time limits. It could be at the top of my list of hardest hacks, but idk, we will have to wait and see when a video is released.