I personally went to Jawbreaker next because I would play it when I got upset at Nine Circles so I got a headstart of 40%. But Fairy dust would result in a new hardest and Jawbreaker's final 30% is really frustrating in my opinion but Fairy dust is much harder. If none of those suit you I suggest going for Future Funk. I've been trying to beat it for about a month now.
Ive been spending a few hours learning jawbreaker and I low-key kinda enjoy the gameplay. I feel like the constant gravity switches is actually rly fun, even though it makes the level a whole lot more difficult