Who is the hardest boss in Touhou and why?

Mari Normal is Satan, Shou I find tough too. I'll put the one I find hardest from each stage soon.

First of all, I want to note this is entirely subjective so obviously this differs per person.

Second, I'd personally say that Junko is actually, but not for the good reasons.

Junko's ridiculous.

Moving this thread into Reimu's Shrine since it's gameplay-related.

For survival purposes? Junko, Kagerou, Remilia, Byakuren.

Junko in particular has some really crazy random stuff that requires either a lot of luck or expert dodging skill to consistently manage. The first non is stupid and tight and even though it's mostly easy, can still catch you at the worst times (i.e. Gobou's 1MNB from a while back). Her first spell is simple sure. The second non isn't even too bad unless she's close to a wall, forcing you to go higher up. The second spell is absurdly stupid because it seems that you can get walled when you're in the corners. I've seen LNN runs and Collet's WR run speedkill it by dodging in the middle of the laser. That's just completely ridiculous and I have no clue how they manage it on such risky runs. Her third non can be nasty if she moves down. The third spell I don't even fully understand, but I've seen it do weird things. The fourth nonspell can just randomly kill you at times even when you think you're safe. The fourth spell is ZUN's being lazy and completely filler. The curvy laser spell is really complex and depends on where the laser waves aim, but it can be quite ridiculous at times. The timeout is easy. And then the final spell, which is just completely ridiculous and I shouldn't even have to comment on it.

Kagerou is probably the hardest boss in DDC, in my opinion, if you take way the glorious homing shots. Her nons are all very wonky and can give awkward situations. The midboss spell is completely ridiculous in the sense that you're supposed to do some quick misdirecting while damaging very tight ice needles and still being able to damage her between misdirections. Some of the dodging can become quite YOLO. The first spell is very tough if you stay in the center to damage rush, since it can pretty much throw a wall in the center. The second spell is completely ridiculous to the point that homing shots just dodge in the middle and end it off in one way rather than going to the corners (like I think you're intended too). You don't have much time to react and it's so random that you can either be really safe or just thrown a clump of weird, fast, and angled glowshit. The third spell is particularly obnoxious because of the red bullets that interfere with the purple bullets while you're in one of the bottom corners. I think even Seija is a little easier than her even with reversed controls.

Remilia is also pretty random to the point where you can get really nasty situations on Vampire Illusion, Scarlet Meister, and Scarlet Gensokyo.

Byakuren is among the lighter of these bosses, but she's still pretty solid with that first spell and Devil's Recitation (I still, to this day, have yet to understand how to even approach DR).

Other honorable mentions are Sakuya with her batshit nons and Killing Doll, Clownpiece for the wonky hitboxes and tight nonspells. Doremy for that stupid first non (And then the following nons that depend on how high she is on screen). Shou with her first spell and second non combo, and Kaguya for having an array of fairly difficult RNG patterns.

For scoring purposes? Er... I'm only really experienced in PCB so no comment. Considering all the factors like different game mechanics and such, it'd be hard to name one individually.

Rumia on easy mode is the hardest boss. I can't even see the bullets!

I have to say, I dipise Reimu on Normal mode, I just seem to die to her all the time...

Depends on who i'm rusty on that day.