Perspective from a Lunatic player:
Easiest
Stage 1: Rumia or Wriggle. Both of them just aren't a threat to me in any single way.
Stage 2: Parsee. Especially when comparing her to the rest of the SA bosses, she is a joke. She has reasons to be jealous. :3
Stage 3: Don't really know. A lot of stage 3 bosses feel pretty equal to me. I would say Hong then.
Stage 4: Marisa from 8. Lot of spell cards but not a single one is really threatening.
Stage 5: Reisen. Only that Loafing Wave spell is a threat (a pretty big one though)
Stage 6: Utsuho or Miko. I find Utsuho to not be that difficult really. Only Nuclear Excursion and sometimes Peta Flare can steal my lives. And although Miko has the most Final Boss spell cards.. none of them are a real pain. Maybe the Royal Dance one (I rarely read the names lol)
Stage EX: Suwako. Enough lives, not many threatening spells, and a Hard/Lunatic player can read the final spell without too many problems. Koishi. Maybe it's because that's the only EX stage+boss I've been trying to perfect but... yeah. I can clear every spell pretty reliably except Genetics and Philosophy.
Hardest
Stage 1: Minoriko, Yamame and Nazrin. Sorry, I couldn't choose again x]. Minoriko because especially her last spell still tends to get me, Yamame because first non-spell on Lunatic is broken or something (the bullets go retardedly low sometimes), and Nazrin because Pendulum Guard spell (although that one doesn't kill me that often, it's just spooky).
Stage 2: Kogasa. CHEESUS DATS AGRESSIVE AS F**. First Spell is very doable with the OP SanaeB, otherwise it's painful. Second spell is not that hard but random factors can mess things up. Third spell is a guaranteed bomb for me, I always ruin it someway, at some moment.
Stage 3: Ichirin. First spell = bomb 4/5 times, second spell = out of bombs, rip. Third one is doable but stresses me out because I just got killed on the previous one and got some bombs back. Also I often mess up Kagerou and Yoshika, dont know why.
Stage 4: Patchy for any mixed ground spell or mixed Fire spell. Also, I never really did Yatsuhashi Lunatic in a full game, but I tried it in practice and it's pretty evil. (Also Seiga messes me up sometimes)
Stage 5: Orin. I can't guarantee at any of her spells that I will clear it. Usually I have to bomb 4 out of her 5 spells (midboss included). Shou is dangerous but in my case her spells are actually quite steady and reliable.
Stage 6: Yuyuko ლ(ಠ益ಠლ)
Stage EX: Mamizou. Although it's the first EX boss I ever defeated (back when I only 1cc'd TD on normal and only had TD, like 2 years ago) I never did it again as far as I know. Every time I try it again I fail miserably.
Also, yey first comment ever on this forum