If we're really talking touhou, 6 is a bad idea. It's one of the hardest games- I'd say it's only easier than the 11, the 12 and the 9 (with all of the photo games aside. Those are insane). You also can't see your own hitbox in the sixth, which is a big disdvantage to a newcomer (after you get better it doesn't matter all that much, though). 8 is the easiest, though it's still plenty hard.
Even if you don't pick the 8, I'd suggest the 7, the 10 or the 13.
Your thoughts on this song?