I nominate Donate. Nothing like this had come along that gives your opponent anything on your side of the field. Looking beyond the whole Trix deck that was dominant for a good while, wouldn't you love to donate your carnival of souls to your opponents?
It became a killer tournament card because Illusions of Grandeur was just sitting around and waiting for a way to be forced onto an opponent. But Donate's uses outside of the "Trix" combo have almost always been very casual. It provided a way to do something cool that simply hadn't been practical without it. Here is an example: http://www.casualplayers.org/article/get.php?action=getarticle&articleid=603