Library of Alexandria would probably be my most wanted card if it weren't for the fact that I hate them.
Its way too abusable in so many different decks. It was a mistake of a card and is infinitely more powerful than any other land ever printed (although the Port can somewhat control it). I have seen this 'land' draw over 10 cards a game too many times for my tastes. Keep in mind that this thing produces mana, does not come into play tapped, and has no drawback (like the City of Brass). WTF.
It is a super-mutant in the world of Magic.
I feel better now, so I'll say the Abyss.