>Where should I start?
The games until 5 were designed for the PC-98 platform, which would require an emulator. It's recommended you start either with the sixth (first Windows game) or the second game (as the first one isn't a bullet hell) and work your way up from there. Some games aren't your usual bullet hells but should be played anyway.

1: Arkanoid's drunk abusive cousin.
7.5, 10.5, 12.3, 13.5, 14.5, 15.5: Fighting games.

The others follow the basics of the bullet hell genre but usually add a game play twist. Once you're done with the main series you can move on to the fangames, of which some are of excellent quality.

>Touhou Project Official Material Reading/Playing Guide
https://m.imgur.com/s1JmZu8

>eraTohoK (plus other era games)
https://pastebin.com/6CULQcKd

>1cc charts! (Updated for AoCF)
https://imgur.com/a/Mkl4Q

>PoFV and Soku Netplay Guide! (Needs your help!)
https://imgur.com/a/ZCGuE