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?
Fodder Cannon!
This is the 2nd best flavor text in all of Magic, and I hate flavor text. Flavor Text:
Step 1: Find your cousin.
Step 2: Get your cousin in the cannon.
Step 3: Find another cousin.
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
I nominate this card because:
1.) He's a big fat green critter and who likes big fatties more than "Casual" types?
2.) I've seen him alot in decks played by "newbies".
3.) He's GREEN for Pete's sake!