The reason i prefer fighting Aya is that she have some better spellcards than the Prismrivers imo. Aya's non-spells are pretty meh though. After re-reading my initial post i reached the conclusion that it was a little too harsh than whats fair. I would say that the game is indeed very good but it just isn't anywhere as good as the other Touhou games in the Windows Era. 8 out of 10 i would give it if i were to review the game.
I actually had a load of fun playing at least the first three stages in Hard difficulty and the three later stages at Normal. I'm planning on working on the Hard mode for stage 4-6.
I still think the Prismriver battle sucks though. Especially when compared to Patchouli and Satori.
About Youmu. My friend always tells me how her spellcards aren't that bad but her non-spells are the toughest about her. I disagree. I almost always absolutely ace those attacks. Youmu's openers feel like trivial dodging to me and so does Delusion of Enlightenment and whatever the Hard mode version is called. But when she uses slow-down i get hit. No matter what.
I don't agree with anything else, especially since PCB has the Crowning Moment of Awesome right at the end that no other game in the series can claim, with the possible exception of that one with Virtue of the Wind God Mountain of Faith.
PCB's conclusion is definitely awesome, i'll give you that. But i think VoWG is a hell of a lot more fun than X% Reflowering thus making MoF's conclusion better IMO.
Erin is by far the worst of the final bosses, imo, while Remilia and Yuyuko are the best.
There is only one reason i forgot about Eirin. I never play against her. One exception. I was doing a no-bombs run of IN the other day and i decided to play against Eirin for a change instead of Kaguya. I then realize why Kaguya is the better boss. Eirins spells sucks!!! Almost all of them are very easy even at Lunatic (at least compared to other final bosses) and if you are going for score there is a lot more potential for that if you play against Kaguya.
A good argument for fighting Kaguya could be Apollo 13 but Kaguya have LSI so...
As for who's the best final boss i still think its Yuyuko (because i don't feel Eirin counts.) while the best would be Remilia or Kanako.
On final and this time i mean it, final word. I'm happy you gave me some good replies to my topic. I feared i would get flames and angry posts. Maybe i'm just paranoid.