The Jack Mullins wagon is my favorite. But, since it has been mentioned several times, I'll add another favorite, which is the Tons-a-Fun '72 Lemans 455 Pontiac powered wagon--or I should say wagons. Gary & Charlene Wood actually had 2 of 'em. One had a regular front cap. The other had a GTO type cap.
http://www.hotrod.com/articles/0212hpp-pontiac-history/