Emrakul, the Aeons Torn

The first of the original Eldrazi titans, and the one Wizards built as the hardest stop sign in the game. Read the abilities not as a stat line but as a stack of failsafes: it can't be countered, so blue cannot answer it as it resolves; protection from any spell that is one or more colors, so nearly no removal can touch it on the battlefield; a free extra turn stapled to the cast, so even a chump-blocked attacker still buys its controller a full untap, draw, and swing; annihilator 6, so blocking is irrelevant once the defender has nothing left to keep. The graveyard-shuffle clause is the most telling choice: putting it in a graveyard doesn't bury it, it reloads the deck, which is exactly why it became the payoff for self-mill and library-management strategies that wanted to deck themselves on purpose and never run out of cards.

The 15 mana is not a number meant to be paid; it is a fiction large enough that a hardcast Emrakul is a thought experiment. The extra turn keys off "when you cast this spell," so the cheats that skip the cast forfeit exactly that reward and leave a 15/15 flier with annihilator and full protection. Even cheating it into play is its own puzzle: the shuffle trigger fires the instant it touches a graveyard, so sorcery-speed reanimation can't catch it. You need an instant-speed answer like Goryo's Vengeance to grab the body before it shuffles itself back home.