There is a reason Seihou never became massively popular. Objectively seen, Touhou Project is overall better, and only the Touhou cameos gave Seihou anything resembling popularity. That being said, I believe that you should give Seihou a shot, if only for the music.
Shuusou Gyoku
Shuusou Gyoku is, objectively seen, decent at least. It has high points, such as the Stage 2 boss fight, which is easy, but still a blast to play. It has low points, such as over-reliance on VIVIT's very tiny hitbox and terrible graphical presentation at parts (bullets hard to make out), as well as really damn bad sound effects. Some boss patterns are also rather uninspired, and not that many bosses overall have a theme of what they are doing. Milia uses fanning laser spreads and restrictive bullet chains. Mei&Mai use bullrushing attacks, sweeping lasers and cover-fire. But what's Gates do? You sometimes miss the "signature" moves, which only were codified by Kioh Gyoku. Speaking of which...
Kioh Gyoku
This is better than Phantasmagoria of Flower View. The balance is not perfect but there are no god-tier characters (Shiki/Komachi hurrrr), the basic game mechanics, particularily the emphasis on grazing, are well thought-out and the game plays smoothly. Characters are well-defined, have their roles, have distinctive signature moves (I could say what every character does in Kioh... I can't remember most of Po FV's stuff.) So overall, this is the most well-designed Seihou game.
But all that will probably not matter much if you are restricted to single-player. Story mode is a joke with a rather fixed chain of five bosses and only the last two matter plus three unlockable characters - that's four playthroughs, which probably won't take you more than two hours if you are not stalemated by blatantly shieldhacking Muse.
But assuming you can set up Netplay, you will probably have a blast.
Banshiryuu
C67 is blatantly unfinished but shows promise, with great stage 1, 2 and 5 boss fights and overall well-designed stages. Then the C74 version throws all of that out of the window for ooh shiny graphics.
You still should give C67 a shot, but you better never play C74 yourself, just watch the extra stage playthroughs.
I personally have a special connection to Shuusou Gyoku. It may not be rational, but I still enjoy it.
A series held back by glaring flaws
There is a reason Seihou never became massively popular. Objectively seen, Touhou Project is overall better, and only the Touhou cameos gave Seihou anything resembling popularity. That being said, I believe that you should give Seihou a shot, if only for the music.
Shuusou Gyoku
Shuusou Gyoku is, objectively seen, decent at least. It has high points, such as the Stage 2 boss fight, which is easy, but still a blast to play. It has low points, such as over-reliance on VIVIT's very tiny hitbox and terrible graphical presentation at parts (bullets hard to make out), as well as really damn bad sound effects. Some boss patterns are also rather uninspired, and not that many bosses overall have a theme of what they are doing. Milia uses fanning laser spreads and restrictive bullet chains. Mei&Mai use bullrushing attacks, sweeping lasers and cover-fire. But what's Gates do? You sometimes miss the "signature" moves, which only were codified by Kioh Gyoku. Speaking of which...
Kioh Gyoku
This is better than Phantasmagoria of Flower View. The balance is not perfect but there are no god-tier characters (Shiki/Komachi hurrrr), the basic game mechanics, particularily the emphasis on grazing, are well thought-out and the game plays smoothly. Characters are well-defined, have their roles, have distinctive signature moves (I could say what every character does in Kioh... I can't remember most of Po FV's stuff.) So overall, this is the most well-designed Seihou game. But all that will probably not matter much if you are restricted to single-player. Story mode is a joke with a rather fixed chain of five bosses and only the last two matter plus three unlockable characters - that's four playthroughs, which probably won't take you more than two hours if you are not stalemated by blatantly shieldhacking Muse. But assuming you can set up Netplay, you will probably have a blast.
Banshiryuu
C67 is blatantly unfinished but shows promise, with great stage 1, 2 and 5 boss fights and overall well-designed stages. Then the C74 version throws all of that out of the window for ooh shiny graphics. You still should give C67 a shot, but you better never play C74 yourself, just watch the extra stage playthroughs.
I personally have a special connection to Shuusou Gyoku. It may not be rational, but I still enjoy it.