This is a difficult choice for me. People have pointed out some extremely good candidates for a limited run re-release (Buzz, Masterpiece, Big Kahuna, Guaranteed, New Standard). I can't fault any of them as they are all excellent pieces.
However, I still have a soft spot for the original World Class - as it was the first ball I bought from LM NIB, and would really like to find two of them to add back into the collection. (Ironically enough, it's just about the only 2 piece ball LM made that I no longer have at least one of - that and the Guaranteed and Kong)