>>8932I'm gonna go off the board and say either "Imperishable Night" (TH08) or "Undefined Fantastic Object" (TH12).
I do admire "Embodiment of Scarlet Devil" (TH06), but it's a bit too cliché to answer with that and the game doesn't even have a hitbox when you focus, while "Imperishable Night" (TH08) is the most polished of the early Windows trilogy and has a wide cast of characters. If you absolutely suck at danmaku games, I believe that the community consensus is that "Undefined Fantastic Object" (TH12) is the easiest game.
Overall, ZUN tries to make each Touhou game standalone and avoid continuity (even though there is some if you dive deep enough) so it'd be easy for newcomers to simply jump in at any point.