When I was growing up G.I. Joe and Transformers were a lethal one-two punch for my imagination. The toys were cool, the cartoons were awesome, but the box art was the catalyst for daydreaming. Seeing that stuff on the shelves, looking at the hand-crafted portraits of the characters really got my mind going. The fact that these were actually painted, with all the textures and imperfections that come along with that process, really drove home a certain personality that is all but lost on kids today.
You can scope the entire series of box art over at Botch the Crab.