I nominate Conspiracy. It probably isn't as good as it used to be and was not among the most powerful of cards even in this relatively innocuous set, but there was and is (in some decks) great demand for what it can do. There were already some creature types you could build decks around (slivers, goblins, etc.), but I think Conspiracy was the card that first introduced the possibility of building a deck that could be based around ANY creature type. Conspiracy was never broken, but had synergy with a long list of cards in every color and style. I think that's the mark of a fine casual card.
Plus you could name walls and Donate it to your opponent so that they couldn't attack. That's pretty cool.