It probably has a snowball's chance of winning, but I nominate Tendrils of Agony. I might be a bit biased...
The storm cards, but mostly this one, changed combo strategies forever. And casual players can use combo decks too. I loathed storm when it first came out, because I liked playing more traditional combo decks and the change was so drastic.
Before Scourge, the idea was to get two or three cards into play that all worked together to have some powerful effect. But with Tendrils, that strategy became obsolete. Rather than stringing eight spells together all to put two more on the table that could win you the game, the "combo" became just one card fueled by the spells that came before it. More efficient, harder to stop, and the possible combinations to get the lethal Tendrils out there are virtually endless. Really, it doesn't even need to be a lethal tendrils. You can hit, gain a bunch of life from the Tendrils, reload, and play another or kill with some other card.
It has been used as the primary kill in some very broken decks, but I would argue that things like Yawgmoth's Will and Mind's Desire are the broken elements there. Tendrils is just an effective finish for them.
I'm also going to nominate a dragon-themed enchantment, Form of the Dragon. For the first time, Magic players got the ability to "experience" the game as a creature. Not just any creature, either. You got to be a fricking's 5/5 flying Dragon!!!! How many of you how every wanted to have the ability to shoot fireballs out of your mouth, laying waste to your enemies?!..... huh? Just me? Oh well. It's still the most casual card.