IMO, Night of Nights is probably the best touhou song
Do you mean the actual Flowering Night, or the arrangement?
Yes but necrofantasia is the hardest.
Native Faith, Reach for the Moon Immortal Smoke, Voyage 1970, amongst others would like to have a word with you.
Don't get me wrong, I do believe Necrofantasia is a complicated piece, but definitely not the most complicated.