Now we're into the top 3. While everybody I've seen on the forums has accurately predicted the top 2 (though the order changes from person to person), the 3rd place card differs depending on who you ask. For me though it's this year's most powerful Synchro monster, which quickly became an Extra Deck "staple" no matter what Deck you ran.
Trishula, Dragon of the Ice Barrier(Hidden Arsenal 4: Trishula's Triumph)
1 Tuner + 2 or more non-Tuner monsters
When this card is Synchro Summoned: You can banish up to one card each from your opponent's hand, field and Graveyard. (The card in the hand is chosen at random.)
Ever since Synchro Monsters have come into existance there had been situations where you'd have 3 monsters on the field, and there'd be nothing in the Extra Deck that could handle what your opponent had laid out infront of you. "Man I wish I had a Level 9 Synchro right about now!" was the call of the Duelist in this situation, and while Mist Wurm solved that when it was released in late 2009, it wasn't exactly a card that every Duelist needed to have in their Extra Decks.
Trishula changed all of that when it was released in its own booster pack earlier this year (when a booster pack has a card name in it, you know that card has to be good) and suddenly every Duelist wanted their Deck to be able to bring out The Three-Headed Frozen Dragon of Doom. Trishula was so powerful that people built FTK Decks around it, and if you haven't seen "Bahamut's Divine Performance" yet you must click this link right now.
When Trishula is Summoned you can banish up to 3 of your opponent's cards; one from hand, one from field, and one from the Graveyard. Since the in-hand card is chosen at random the entire effect doesn't target, which means that you banish cards after your opponent responds with cards like Bottomless Trap Hole etc. to try and take Trishula out. Given how Graveyard-orientated the recent formats have been, a well-timed Trishula can spell (frozen) doom for your opponent unless they have the Warning/Veiler that can save them. As a Plant Duelist you never want to lose your Dandylion, Debris Dragon and Spore, and potentially losing all 3 at once is devastating. Same for Agent Duelists and losing Hyperions and the Agents needed to Summon them.
There aren't many Decks out there that can stand up to a Trishula that's unleashed its rage on them, and even the ones that can will have a tough time getting back into a Duel without getting hit by the dragon's 2700 ATK at least once. So long as the card is legal it'll be used, it'll be Summoned and it'l be feared, which is why it's totally deserving of its spot in 2011's top 3.
(On a side note Trishula is my personal favourite card of the year. It'd be higher on the list, but the top 2 have made a bigger impact on Duels this year than several other cards combined.)