Whoever was closest, while grabbing on to anyone else nearby so that they won't run away. Then I'd move on to the next one. Then I'd seek them all out and impregnate them in the order I encountered them in.
If I would encounter several of them at once, and more than one was equally close to my dick, I would do the one that my dick was pointing the most at.
If more than one - usually two - was on either side of my dick, with my dick pointing exactly between their pussies, I would grab them both, and the one that I could position the quickest, would be impregnated first.
If they, in the above situation, collided exactly midway, so that I still couldn't decide, I would cry tears of sorrow as my heart tore in two. I would collapse into a pool of tears, and all the Touhou girls would slowly back away.