Hardest:
Stage 1: Obviously none are much of a threat even on Lunatic because Stage 1 label just holds them back a lot lol, but I will personally say Nazrin - Some of the spellcards are worthy of a stage 2 in any other Touhou IMO.
Stage 2: Kogasa! Yeah, yet another one for UFO. Again, those spellcards seem like they would usually belong in a slightly higher stage than were they actually are!
Stage 3: Keine. A fair difficult jump from Stages 1 and 2 in any difficulty, particularly in Lunatic.
Stage 4: Ummm... Whatstheirname from DDC? I Dunno, I honestly cant decide, I would go with (Them) just because I have not played DDC that much, compared to the other games.
Stage 5: Orin is a worthy opponent... fairly annoying spellcards even on Normal... yeah.
Stage 6: Sukuna, EASILY. Once again, it might be simply because of my lack of practice with DDC, but I just don't really like much of her fight. Also, "You Grow Bigger"- Destroyer of what (WAS going to be) a first play 1cc Normal blind, and a HORRIBLE idea for a spellcard, not to mention Hitbox issues with anyone not named Reimu, can go to hell
Extra:
KOISHI.... with absolutely
NO DOUBT AT ALL. She seems to be more than Extra, she seems like the boss that actually deserves the label "Phantasm" (Sorry Yukari you are awesome but your fight just wasn't particularly hard after Ran)...
Its worth noting that I have yet to beat Ten Desires on Normal (Yeah I know. Its somehow the hardest Normal clear for me, considerating I have already 1cc Normal cleared all other Touhous), so I don't know about its extra, but I highly doubt it could possibly be harder than Koishi!!!!!
Easiest:
Stage 1: Rumia... She is the very first enemy in the Windows series, so I guess it makes sense that she is the easiest one.
Stage 2: Chen. Theres just not much dodging in her fight...
Stage 3: Alice... for a Stage 3, fairly easy-ish and memorizable fight. Not much of a threat, even though she comes back like 3 times during the stage (lol).
Stage 4: Prismriver Sisters (Yep, theres a pattern going on here); Fairly easy for a stage 4 boss... I mean cmon on, you are 3 against 1 and your first team spellcard consists of STREAMING... Lyrica for Lunatic is trivial, so is Lunasa in Normal, and the final 2 cards aren't that hard either, and since they are 3, they are extremely vulnerable to bombs.....
Stage 5: I guess Reisen. I personally think she is actually kind of hard, but I can see how memorizable her fight can be...
Stage 6: Yuyuko (DEFINITIVLY a pattern!)... Literally the ENTIRE fight is memo/streaming... Theres barely any random elements to make it challenging. Only problem is Reflowering, which isn't THAT bad once you get used to it. Plus I read somewhere you can abuse borders with it somehow.
Extra: Ran (Yeah there might be some bias) IF you know the tricks to each spellcard, it becomes a fairly trivial, if lengthy fight. I realize that's true for every Extra ever, but more so for Ran, IMHO.
I tried to be as non biased as possible... but, theres only so much you can do when you have 100+ Hours in a certain game (In my case, PCB, so that's why 5 out of 7 on Easiest Bosses are from PCB), while the 2nd most played Touhou is maybe on 30+ Hours... LOL.