Which Game has your Favourite OST?

PC-98: Quite good, just underrated in general. HRtP is solid for ZUN's first work. PoDD is already a strong balanced contender. Yuuka's themes and Alice Maestra (foreshadowing MS Alice?) overshadow most of LLS, are stylish but could be made a bit better. MS compositions are well-made and are a nice finish to PC-98 era. Shinki's theme is a bit bland for a final boss but Alice has very nice tracks both in main game and Extra. If you count Romantic Children, that's a lot of Alice. Good that ZUN has saved her from PC-98 cataclysm.

EoSD: Firstly, underrated title theme. EoSD is famous for having a lot of so-called "meme songs". Those are well-known by majority of people that have encountered Touhou. ZUN tried to go for a Western feel, and it shows so; though preserving a nice style difference between individual ones. Tracks are not so well-polished yet but ZUN was not yet well-accustomed with making music using Windows. Nice MIDI's though, good soundfont can really make a difference.

PCB: Very soft music, the softness maintains throughout the whole "album". More Alice. My favourites are stages 4 and 5; PCB has very strong stage themes in general compared to most games/PCB's own boss themes. Ultimate Truth is good for its' length. Extras sound very youkai-ish, which makes for a good shift from main game.

IN: A nice load of great piano compositions, too bad that a half of it is impossible to play by a normal human. Best stage 1 by far. Very strong title. When I opened IN, from that title alone I knew I would love it. Reimu, Keine, Mokou, both Voyages, Eirin, Kagome Kagome, Reisen, Last Words... Playing IN feels like experiencing a dream more than in any other game. That Gensokyo-ish daydream feel is better presented in IN than anywhere else. On the other hand, the songs that don't take advantage of ZUN's ridiculous piano abuse combined with being drunk all the time are quite mediocre (Love-Coloured Master Spark, Kaguya, bits of Extend Ash)

PoFV: All "remixes" are meh. Wind God Girl starts out great but it has those ridiculously high-pitched zunpets at the end which kinda spoils the piece. Yuuka's theme is quite a bit worse than both of her PC-98 ones, which is a shame. Not a fan of Medi's, either. However, both Komachi and Eiki are great. Given how long Eiki's fight is (almost half of a game if using a bunch extends) making just as long and powerful theme was a necessity. Fate of Sixty Years is one of my all-time favourites. One of reasons to play PoFV is to experience battling a worthy opponent, a highly-powered Yama, for several days, then trying to survive as much as you can in an inevitably lost first round... then to take control of such Yama and lay down some punishment on that lazy, hax-using, having-way-too-much-innate-power shrine maiden on your ground, while your epic-ass judgement-screaming theme is rocking.

MoF: Deliciously Japanese-sounding OST. Fresh. Noticeable difference from the classical trilogy (6-8). Clearly themed after flashy Shinto festivals. Not much to say, you have to listen.
SA: Even though the game features a lot of fire, the themes are a bit cold and evil. That's not a bad thing, just that it's a bit hard to listen to them casually - those are themes fit for confrontation, representing underground creatures rather well. Orin - more like mischievously evil compared to others, but it's still one shade of color. The OST is mostly your typical combat music in ZUN's style but it's done well, especially Nuclear Fusion. Last Remote is a nice break after these fights, reminds that it's still winter above the ground.

UFO: Most are good. Extra stage is a bit bad but a nice try. Heian Alien has a nice bit in the middle, and is very fitting. Most themes are quite suited for the situation - raiding the Palanquin ship, glorious Makai entrance (long time no see!), Shou (theme is well-made but is a bit quiet and reserved, so people don't often pay attention to it in such a hard fight), whole Fires of Hokkai -> Cosmic Mind chain. Byakuren is basically Shinki done right; such a hard and powerful boss with flashy cards and long backstory deserves that theme, and Fires of Hokkai make a good "preview" before her. It takes some time to develop, but later half of the fight is made ten times better by it.

TD: Feels like a darker MoF. Last third of the game is much better and more rememberable. Stages 5 and 6, teh boat, Miko. True Administrator could be better if ZUN chose other instruments/samples; for some reason a piano arrange makes it sound better than the original. Epic theme but doesn't feel as epic when it's presented in game itself (f.ex. compared to DMBN's arrange/s)

DDC: Raiko's theme is best Extra theme, incredibly fitting to the whole fight, good pacing. The OST gives a nice feel of "good old times", starting from the title. Definitely has a PC-98 feel. Overall solid besides first two stages. Illusionary Joururi feels harsh at first, but opens up a little after initial bit. Shining Needle Castle Sinking in the Air is the best stage theme out of newer 3 games; poignant like PCB stages but with better, modern quality. Sukuna could be a bit better but is not that bad.

LoLK: Music is clearly the better part of the game. Not a fan of stages besides maybe 4, and 6 is more like a buildup to Junko's theme than a piece in its' own right. ZUN wanted to make an epic final boss theme comparable to Yumemi and Eiki, and he (mostly) succeeded. Pointdevice gave him an excuse to finally do a lengthier theme again. Doremy's theme was a highlight since the trial, and stays as my personal favourite of the game. Sagume fight was well done. It's short and the theme was overlooked by most, but I like it.

Spinoffs: DS/StB are top tier as said above. ZUN applied good effort trying to make the OST for those quite refined. GFW is okay. ISC has good Day 1 which represents Seija nicely, but then the album starts to drop off. As far as fighting games go, IaMP's night disk alone beats Soku (which only really has Iku, Tenshi, Ridiculous Game and Unknown X, good but not exceptional by ZUN's standards), while Evening Star and Missing Power are strong on their own and serve as nice illustrations to Yukari and Suika respectively. ULiL deserves a honorable mention for Miko's remix, Kasen, Usami; so is HM for Lost Emotion even though that one is fairly repetitive. Those tracks feature nice experiments.