Probably HRTP, though I've never played it. It just doesn't seem like that good of a time.
VD is really jank, and HBM is close behind it. Though I find HBM's jank slightly charming in its own way.
SoEW is also extremely jank and the worst of the 4 PC98 shmups.
So those four games probably.
I don't really agree that the other scene danmaku games were that bad, stb and DS are both pretty nice.
As for music, I want to apologize for bashing 17, since that one at least still HAD a style, whereas 18's is about as generic as it gets.
18's music definitely does have a very distinctive style. I would describe it as very "fresh" sounding, he's switched to tighter drums, and really dialed up use of more unusual instruments like guitar, flute, erhu, and voice. People call it more repetitive, but I just think modern tracks tend to do more buildup, and thus fit better when listened to in the context of their game (which is all ZUN cares about, and the reason why he never actually releases plain OST CD's for the games: the game
is the OST)
In general, 18, 18.5, 19, and oneoffs like Every Day a Red Day and Independent Simulacrum all define ZUN's modern style pretty well, though it's still evolving continuously.
18's OST has a couple duds (namely the two Stage 2 themes), but many other themes left a pretty strong impression on me -- Stage 1 is another short and sweet banger with an earwormy melody, Stage 3 is one of ZUN's only major key compositions, Stage 4 has the Hifuu-like flamenco guitar, Stage 5 meshes with the stage background and presentation to really sell the sunset setting, and Extra is probably in my top 3 extra stage themes due to how hard it goes.
After 18, all I can do is wonder why anyone even bothers with what's going on with the series anymore.
Ouch. Good thing we can bother, and you don't have to :P