Eirin vs Kaguya~

I'm gonna look at this from multiple different aspects.

Based on characters, Eirin is in my top ten while Kaguya has consistently secured ABSOLUTE BOTTOM placement. I'm starting to appreciate Kaguya more now, but she's not going anywhere near Eirin tier. Even based on appearance, Eirin has a cooler outfit and I like that silver hair. And she holds a bow. Kaguya looks really bland in comparison.

Based on boss design though, there are a lot of factors to consider. With respect to spell design from a gameplay perspective, I like pretty much all of Eirin's spells except Apollo 13 (which is still not too bad), whereas the only spell designs that stand out for Kaguya to me are Fire Rat's Robe, Rainbow Danmaku, and her 3rd Last Spell. Stuff like Dragon's Necklace, Buddha's Stone Bowl and Swallow's Cowrie Shell, although they are very different spells, aren't much more than just different variants of mixing lasers and bullets, which is kinda lame. Fire Rat's Robe technically falls in that category too, but its unique characteristics (destructible familiars firing aimed lasers from a rectangle formation) improve its overall design more (despite being quite easy). In any case, Eirin beats Kaguya here, even if Earth in a Pot and Galaxy in a Pot were treated as a single spell.

I also especially like how Eirin's spells tend to have a recurring theme of being surrounded or confined. Her spells well demonstrate how Eirin has complete control over your freedom of movement, and I personally find that this really helps to illustrate how dangerous Eirin is as an opponent, which fits her nature as a final boss. Kaguya doesn't really have any kind of notable recurring theme like that, sadly. Still though, with respect to visual presentation, Kaguya's spells are prettier than Eirin's imo. Also, while not really related to Eirin vs Kaguya, I have to point out that Eirin's emphasis on spell design and Kaguya's emphasis on visual presentation do a good job of reflecting their individual distinct characters.

While not really a very important factor, imo History of the Moon > Flight of the Bamboo Cutter, and I'd say the former also fits better as a final boss theme.

I'm not really considering the difficulty of the battles as a contributing factor, but I'll express my opinion on this anyway. Eirin is tougher on Easy/Normal (Apollo 13 is harder than anything Kaguya throws at you, Game of Life is a bit annoying at times, and even Astronomical Entombing can be pretty difficult, depending on your shot type) while Kaguya is tougher on Hard/Lunatic (mostly because Hourai Jewel becomes insane).

Both bosses are fun, though I find Eirin's battle more exciting because it tends to be pretty intense throughout the fight, whereas fighting Kaguya is pretty relaxing until like, Hourai Jewel. It doesn't help that Kaguya's hardest spell on Normal imo is her first spell, Dragon's Necklace. Kaguya's Last Words are also too short to compare to the tense experience with the longer Hourai Elixir, though failing at less than 5 seconds left on the latter is frustrating as hell.

My vote goes to Eirin btw.