Tough question! If I could only do one race on this theoretical trip, I'd have to take a race at Spa-Francorchamps. Its definitely the track that I most enjoy watching on tv, and, to me at least, has the most history and mystique. Eau Rouge and Raidillon has to be the best section of track in the world.
However, if I was allowed to sneak in more than one track, I'd have to make a lifelong dream come true and head "down under".