Oh hey, a thread about nonspells, neat idea :3
Lemme see... I'll be mainly talking about the Normal mode nons here.
So far...
Hardest: Clownpiece out of everyone takes the biggest slice. Her first and second nons keep screwing me over on a regular basis, only possible way to get through is either to have godlike reflexes, Reimu's hitbox or Reisen's triple-shield bomb. Then comes Junko with her 3rd stage 6 non and both of her Extra nons, their density is absurd, it's near impossible to properly navigate through them unless you're using Reimu and make extremely precise movements. Orin's last two and Byakuren's last two nons tend to take most of my remaining resources as well. IN's Marisa takes a slice here too, all of her nons there are really damn tough and a huge difficulty spike compared to Keine.
Most annoying: Youmu and Lunasa. While not necessarily very hard, they're really dense and navigating through them is kinda a pain in the butt. Also there's Benben with her first and third nons, the randomness in the former and the tight squeezes through the slow musical notes while watching for fast ones in the latter tend to wreck me too. Raiko's fifth non and Yoshika also have a share here.
Most interesting: I've been taking a considerable liking to Doremy's spiraling nons, their patterns are simple but yet appear to be very elaborate and kinda elegant, mainly because of how they're alluding Doremy's ability being dreams-related and the fact that spiraling visual effects rather have an hypnotic effect on people and make them sleep, which I think is pretty clever. Sekibanki's nons, while a bit underwhelming in terms of difficulty, seem quite interesting to me as well, because of her being a rokurokubi/nukekubi-type youkai and the bullet trails coming out of her resemble both stretched necks and multiple heads flying away from her leaving a trail behind.
Most fun: Ringo's nons are a thrilling grazefest. Patterns are simple but they're kinda fun to dodge. Otherwise I somehow can't really recall any specific pattern that sorta stands out. I basically find all nons more or less fun, even if they're listed in the first two categories.