Thanks for doing these giveaways! I'd have to say my absolute favorite was computer classes, though those were actually not really official yet. (I've been around for a bit -- when I was in HS, the school had just gotten computers in and didn't yet have a formal class structure. Those of us interested were doing after-school sessions, pretty much learning alongside some of the teachers.)
To choose a 'regular' class, I would go with Geometry. It's like a whole class on doing brain-teaser puzzles -- had a lot of fun there.
