Eiyashou.

I love playing all of them in equal measure because every game is just 40 minutes of picking up and playing and absolute heroin on lunatic, but Touhou Eiyashou is the one with the most everything.

Four playable teams of characters
With unlocked feature of playing with all eight of them solo
Which means there's now twelve options to play the game with
Two end routes, one of which is mandatory on only one difficulty for that team before unlocking the true end
Three ending screens for every team (Bad, Normal, True)

Bosses have conditions where they unleash bonus "Last Spells" during which you do not lose lives and cannot lose bombs, you just get an extra go at the character's hardest spell for bonus points and bragging rights.

Spell practice mode for the first time (so popular fans demanded it back when later games did not have it)
Has every single spell in the game individually (over 200 spells, with individual counter for every single character option in the game, so you'd need to capture every spell twelve times on all characters to be a true completionist)
And on top of the regular in-game spells, also has unlockable Last Word spell practice that contains extremely difficult signature moves from every single character in the game, including the main characters.

That's why Touhou Eiyashou (8) is my pick of the best one as having the most.
Personal favorite I don't have.

perfect cherry blossom <3
does not has most content, but... i simply love it
newbie-friendly too :3
edit: changed my opinion, th16 is peak

7 for sure, followed by 6

No, I'm not blinded by nostalgia, I swear