What is the most expensive MTG card ever sold?

I'm curious, what is the most expensive Magic: The Gathering (MTG) card to have ever been sold in the market? I've heard rumors of some incredibly valuable cards that have fetched astonishing prices, but I'm not sure which one takes the crown as the most expensive. Is it a rare alpha or beta card? Or perhaps a special edition or promotional card that was only available in limited quantities? I'm eager to know the answer to this intriguing question.