Wow, it's hard to narrow it down, so I'll tell you all my favs.
Off Kilter is always a must in Canada.
France has a great movie, and don't miss the pastries. Also, they have a great mime/comedian/balloon guy that we can watch over and over again.
Kaki Gori in Japan is (IMHO) the best snack in WS.
Italy has the Living Statues, really amazing.
America has the Voices of Liberty and American Vibe, don't miss them. I've seen all their sets in one day.
Acrobats in China.
Perfume in Norway.
Okay, these are all the things I can't miss when going to WS, hope it helps.