Picking the best Touhou game is tough because it depends on what you're looking for, but if I had to choose, I’d go with Touhou 6: Embodiment of Scarlet Devil. It's the one that really put the series on the map and is often considered a classic.

It's got an iconic soundtrack, well-designed bullet patterns, and a lot of character depth in the story and the bosses (especially Remilia and Sakuya). Plus, the difficulty curve is pretty balanced for newcomers while still offering a challenge for veteran players.