Honorable Mention Necropotence For three black mana, Necropotence gives you access to more cards than just about any other permanent in the game
Rhoda, Geist Avenger + Timin, Youthful Geist I applaud Wizards for expanding the design space around tapper effects in blue and white, but they're often just not strong enough to justify an entire deck themed around them
Mana Crypt is simply unbelievable on rate, so as long as you are careful, its easy to reap the rewards from the additional mana provided and win the game before the drawback becomes relevant
Sakashimas Protege Sakashima's Protege clones your opponents best creature with the double backup of cascade and having its own power and toughness (as opposed to typical Clones that die without something to copy)
When this creature dies, each opponent loses X life and you gain X life, where X is its power
Nekrataal Everyone knew that we needed a cleaner Nekrataal, but we never got one