He-Man and the Masters of the Universe - Video. I still love the theme music to this one. And who hasn't lifted a pretend sword above their head and shouted "by the power of Greyskull!"? Come on, admit it.
Dungeons & Dragons - Video. This one probably single handedly turned me into a nerd. Ok, maybe not, but it definitely piqued my interest in fantasy, D&D, Lord of the Rings, etc. and without all that what kind of nerd would I be? I have to nitpick though that the "acrobat" is the worst D&D character class ever. I mean, Barbarian, Mage, Ranger, Thief - check, check, check, check. But acrobat? That's just out of place.
Thundercats - Video. Japanimation at its best. In grade school we used to play Thundercats and pretend to be different characters as we ran around the school yard. Thunder thunder thunder thundercats!