Since we're talking about bosses, which boss could come alongside your most wanted?
With my most wanted being Reimu, I have some choices. Some obvious ones would be Remilia, Flandre, Yukari, or dare I say her name,
Mima. While Remilia and Flandre are certainly up there for iconic Touhou bosses, having the Touhou boss being a vampire while we already have Dracula would be a bit redundant even if their boss fights wouldn't be alike, wouldn't it?
So it would leave to have Yukari as a Touhou boss as the most obvious choice, and she could be a very fun boss to fight, with her using her power over boundaries, notably her gaps to screw with the screen, teleport, rthrow trains at you, this kind of things a reality warping boss would do. She would be really tricky and fun, so navigating around what she does would be a fun challenge.
Mima is one of the few Touhou characters that would work as a boss who's the personnal nemesis of Reimu, even though she's not relevant to Touhou anymore, and since a very long time. But she could still be a good boss, especialy if she looks like she does in Touhou 2 with giant bat wings. Her attacks wouldn't be like Yukari's and more basic, but still pretty spectacular, since her magic considers of throwing star-shaped projectiles a lot, while she could also swing her scepter for an attack that gets you away from her, so the challenge would be to not only get in, but also stay in.
A more unorthodox Touhou boss for Smash would be Utsuho, or simply Okuu. Basically, she has the power of the sun god and can throw mini-suns at you. And they're not just star-shaped, they're actual suns. So she would do massive damages with every attacks, and they would all be very big, so managing to dodge said attacks and attacking her when she's cooling down or chaging up could make for a very fun boss fight, especialy if we get a remix of her theme that goes very hard, like this metal cover for example