Tied for my favorite in the series with 11. It has some of the strongest general theming, art/character design, and music of all the Touhou games. UFOs, Space Invaders-style enemy placements, etc., all that stuff gives this game a distinct flavor.
Mechanically I feel this is the strongest Touhou game thanks to the UFO system. It recontextualizes every stage - which were previously rather straightforward and uninteresting before (and after) - into an intense scramble for UFOs. Routing is more important now: you'll have to prioritize UFO-carrying enemies, strategically wait for UFO colors to change, decide when to bomb, and decide where to move to maximize UFO bonuses. This kind of system makes the game a lot more intense compared to others, it's really satisfying. Most of the spell cards are pretty tough, but you can get tons of bombs and lives through UFOs, so it balances out.
Mechanically I feel this is the strongest Touhou game thanks to the UFO system. It recontextualizes every stage - which were previously rather straightforward and uninteresting before (and after) - into an intense scramble for UFOs. Routing is more important now: you'll have to prioritize UFO-carrying enemies, strategically wait for UFO colors to change, decide when to bomb, and decide where to move to maximize UFO bonuses. This kind of system makes the game a lot more intense compared to others, it's really satisfying. Most of the spell cards are pretty tough, but you can get tons of bombs and lives through UFOs, so it balances out.