I'm a normal mode player (though I managed to 1CC a couple hard modes).
I'm also not going to include LoLK bosses in my rankings because they would just win most, if not all, of the hardest bosses.
Easiest:
1 - Orange. Her bullets are slow, not very dense, and her third attack doesn't even get the chance to fire anything depending on who you play as.
2 - Kyouko. The majority of her attacks actively reduce the number of bullets coming at you.
3 - PC98 Alice. The only things you really have to worry about are the lasers she fires later on in the fight, but other than that she doesn't really have anything that threatening.
4 - Seiga. She doesn't really have any "that attack" moments, and the bullets are usually slow.
5 - Futo. Easy nonspells, easy spells. Nothing too scary here.
6 - Yuka. Only a couple tricky attacks, mostly her final attack. Miko almost made this spot.
Ex - Mokou. One of the few Ex bosses I've beaten. Was hard to decide between her and Ran, but I think Mokou has a few more easy spellcards. Okina might go here if you use Reimu.
Hardest:
1 - Nazrin I guess? None of them are hard, but Nazrin seems like she has the most ways to get you accidentally killed.
2 - Meira. She has a lot of fast moving bullets and likes shoot from up close. Also, that bouncing ball attack is decently hard. Doesn't help that SoEW doesn't have focus movement or visible hitboxes.
3 - Ichirin. Mostly because of her last attack, but her first spellcard can also be troublesome since some of the bullets fall pretty fast.
4 - SoEW Marisa. Those orbs that circle around her are so annyoing to dodge, and sometimes they will even do their rotation thing in the middle of other attacks, making them even harder to dodge. Wouldn't be as bad if the orbs weren't there for the entire fight.
5 - Satono and Mai. While Shou might have a few nastier spellcards, Satono and Mai more than make up for it by having some of the most annoying nonspells in my opinion, along with still having some pretty hard spellcards.
6 - Eirin I think? This one was tough to decide between her or Byakuren but I think Eirin has slightly more hard attacks. Omoikane's Device, Apollo 13, and Astronomical Entombing all give me a lot of trouble.
Ex - I can't really say for sure since I've only beaten 4 Ex bosses. From what I've experienced, Koishi seemed like one of the more difficult ones though, so I'll go with her for now.