Hmm... I was thinking along the lines of a full fledged analysis or something, but I have a feeling that that won't fly very well.
Remember, effort has to be about equal to the effort needed to write a story. While writing 10-20K for a Medium Pokemon may or may not be challenging to some people, depending on their experience (it's challenging for me. Go away), the quiz should be at about the same level of difficulty, or marginally less.
That said, all I've done so far is state the obvious and agree with a few people. So I'd say Medium Pokemon tops. I think it'll be a bad idea to give out Hard Pokemon or higher left and right, unless the quiz you have in mind is a extremely difficult. Remember, the age range here is large- what seems hard for me is going to be a piece of cake for a bunch of the older (or just a bunch of the other) people here.
Oh well. This is a pretty good idea. I'm in.