Leaning towards PCB, but I'm open to preference votes.
I figure the winner can decide the next challenge.
I totally think it should be SA. That would be the perfect entry point for any new players that wanted to join in!
Really, though, PCB would probably be the best one to start off with. It's probably one of the friendlier games without too many obtuse mechanics. Also, since upandaway mentioned it, do you intend for score challenges to be by stage? That's probably the best way to do it, and it doesn't require a decent chunk of time out of anyone who wants to improve their score and stuff like that.