3 Favorite OSTs (not in order of favoritism):
1) Touhou 5 Mystic Square:
Mystic Square is my favorite PC98 Soundtrack, i feel all of its tracks are pretty memorable, although they are simpler in composition than the Touhou standard. They are not noisy even with their fast tempos, and they go along great with the stages/bosses. It pushes the limits of what FM Music can do.
Well, some of the tracks are pretty boring, like Treacherous Maiden ~ Judas Kiss, at least it's a clear example of the chord progression ZUN uses (VI VII i)
My favorite songs from Mystic Square are: Alice in Wonderland (it's one of the few Touhou songs that changes a lot with the loop, it really feels tragic and epic and the same time. The key change near the end and the deeper FM melody makes it one of my favorite songs on all of ZUN's work, it shows the potential FM music has), Plastic Mind (the melody is somewhat melancholic, combined with its speed, really makes me feel this song represents a mix of desperation and anger, probably symbolizing Alice trying to stop you from wrecking Makai) and Doll of Misery (the drums are great here, overall the melody feels overly quiet almost like it's repressed, maybe it highlights Yumeko being a Maid and probably lacking freedom. It's one of the most complex PC98 songs ZUN made)
2) Touhou 11 Subterranean Animism
Subterranean Animism's soundtrack is a good song after another good song, it's especially darker in tone and fast paced. The drums and bass are very well achieved (something that is usually overshadowed in ZUN's mixes), they help the songs get a darker/deeper feel that helps convey the overall darkness of Former Hell. It also has some good guitars, like in The Sealed-Away Youkai ~ Lost Place or in Last Remote.
My favorite songs from Subterranean Animism are: Last Remote (the tension and release of this track is legendary, combined with the climax, wow!), Lullaby of Deserted Hell (one of the few calm songs in Touhou, i like it a lot due to how it's synchronized with Orin's movements during Stage 5 and the folkish adventure tale vibe it gives me), Corpse Voyage ~ Be of good cheer! (the drums in this one are what i said earlier i liked about this OST, it complements the melody very well as it's rhythmical almost like a machinery moving. When it stops the rhythm at the climax and the guitar kicks in, its glorious.)
3) Touhou 18 Unconnected Marketeers
Unconnected Marketeer's soundtrack shines in how comfortable it is to listen to it all in one setting, as an album the tracks go back and forth from being relaxing (as relaxing you can get with Touhou) stage themes and epic boss themes. You can really sense ZUN's improvement at music making here, every song is good without being boring or repetitive.
My favorite songs from Unconnected Marketeers are: The Perpetual Snow of Komakusa Blossoms (one of the few times ZUN used a mayor key, and it shines thanks to that. The song feels romantic, like a bird flowing through the wind) The Abandoned Industrial Remains (ZUN really nailed the nostalgic feeling here, the acoustic guitar with the notorious bass and the flutes really makes me feel like im looking at something past it's glory days that has a future filled with hope) A Fantastic Underground Railway Network (That feeling of hope gets realized here as you come back into the mine more prepared, it makes me feel like i can conquer the world and the combination of all the instruments in the mix with the well-placed silences is very well achieved)
Least Favorite OST:
I'm keeping it shorter here, as i dislike this particular OST for these reasons:
I really dislike it when ZUN decides to ignore basic music mixing strategies, and he just pumps the ZUNPets to ear breaking volume, when the entire song is just the same repeated thing with no variation or when he abuses the same instruments in the OST. I'm not a fan of repetition in music, one of the reasons I usually like ZUN's music is how he manages to repeat the same VI VII i chord progression without feeling repetitive (like how popular music abuses one chord progression to death, without variation).
Touhou 07 Perfect Cherry Blossom
It breaks my heart to include this one here, but ZUN really dropped the ball. Not necessarily with the composition, as most songs have very memorable melodies, including some of my favorites like Border of Life, The Doll Maker of Bucuresti, Eastern Ghostly Dream ~ Ancient Temple and Hiroari Shoots a Strange Bird ~ Till When?.
Unfortunately, ZUN forgot to add more instruments aside the piano and ZUNPets. He also forgot to add any kind of bass and to regulate the mix before releasing the game. Why do i say this? Have you ever heard Necrofantasia or Border of Life with headphones? It breaks your ears, the ZUNPets are too loud compared to the rest of the instruments, and it really harms those songs (and your ears). It's not the only problem, the OST uses the exact same instruments on all the songs aside Hiroari and all the bass contained in The Fantastic Tales from Tono. When you play this game, your ears saturate quickly from the barrage of the same high-pitched sounds over and over again (it doesn't help that PCB is one of the longest Touhou games)
It's especially disappointing because Embodiment of the Scarlet Devil doesn't have these issues, even if ZUN was an amateur when using Windows and not that experienced with non FM Music.
Honorable Mention: Touhou 12 Undefined Fantastic Object
Don't get me wrong, this is a great OST, all the songs are good. It's only problem is that all the songs use the same instruments (less prominent than PCB), and they sound too similar, it ends up feeling repetitive, even if by separate the songs are amazing. Thankfully, things change musically in the later half (from Stage 4 onwards), with some pretty unique songs like Captain Murasa and Heian Alien, but the first half almost sounds like the same song.