I've been using President's Choice S'mores Kits and they are PERFECT. I was a camp counselor during the summer so I made them in the microwave (graham cracker, chocolate, marshmallow: 30 seconds) for the girls in my cabin on the last night and they turned out absolutely amazing. I may or may not have had 8 s'mores in one night.
Anyways, the chocolate in the kit is milk chocolate (not Hershey's though, a PC brand I suppose) but whatever it is, it is the best s'mores chocolate I've ever tasted. All in all, I'd say go with a thin milk chocolate bar, one that can be melted pretty easily.
Wow, now I really want a s'more (or eight) and it's 12:35AM. Ok time for bed.