I don't know if there's a similar thread already, but I decided to make a topic to discuss who are the hardest and easiest bosses from each stage serial number in the series. So, let's start the discussion!
Easiest:
Stage 1: Rumia. her danmaku are easy to avoid and she has easy spellcard attacks too.
Stage 2: Mystia Lorelei. She has easier spell cards than normal attacks, actually? reducing the vision field helps diminishing the amount of bullets in screen.
Stage 3: Nitori Kawashiro. Her bullets are kinda slow compared to other stage 3 bosses.
Stage 4: Minamitsu Murasa. Easy spell card attacks, the only problem about her is the survival attack.
Stage 5: Sanae Kotiya. She is the easiest for sure, no strategy needed.
Stage 6: Kanako Yasaka. She is quite easy in comparasion to the other stage 6 bosses.
Extra: Nue Houjuu. She is even easier than suwako, many fans agree with this.
Hardest:
Stage 1: Yamame Kurodani. Considering the SA bosses are relatively harder than the others.
Stage 2: Kogasa Tatara. Especially her final spell card, which is pretty troublesome.
Stage 3: Alice Margatroid. She is on par with a stage 4 boss, and it?s difficult to escape the dolls bullets.
Stage 4: Marisa Kirisame. The battle is too prolongated and difficult, too many annoying stars over the screen.
Stage 5: Shou Toramaru. Damn curvy lasers, you need fast reflexes against her.
Stage 6: Remilia Scarlet. Every stage 6 boss is hard, but remilia stay prominent with her danmaku, imo.
Extra: Koishi Komeiji.
Easiest (PC-98):
Stage 1: Sara. LOL, I mean? most of her bullets are not even directed at you.
Stage 2: Kurumi. She never caused me much trouble.
Stage 3: Elly. Especially the first part which she spams easy to dodge tiles.
Stage 4: Yuki & Mai. Regardless of who you beat first.
Stage 5: Yuka. Differently from her awakened state, she is much less difficult on stage 5.
Stage 6: Shinki.
Extra: loli alice. The battle is prolonged and tiresome, but she offers less problems than the others PC-98 extra bosses.
Hardest (PC-98):
Stage 1: Rika. Yet, still pretty easy.
Stage 2: Luize. Lack of options.
Stage 3: The 5 Magical Stones. Mostly Because the boss battle is poorly programmed
Stage 4: Reimu Hakurei. Not as hard as her IN counterpart, but still quite tough.
Stage 5: Yumeko. Definitively one of the toughest stage 5 bosses, with fast paced blades and no patterns.
Stage 6: Mima.
Extra: Gengetsu. Probably one of the hardest bosses in the series.