What is Magic: the Gathering's best card? That's a complicated question; with more than 20,000 unique cards in the game, there are a lot of options for the 'best' card. On top of that, there is going to be a 'best' card in each Magic: the Gathering format because different cards are legal in each one.
Even within a format, the debate of the 'best' card, is rarely settled. Sometimes the conversation can also be colored by Magic: the Gathering's new rules making individual cards stronger. That said, looking at Vintage, the format where Magic: the Gathering players can play anything, there is one card that settles out on top.
Magic: The Gathering's most expensive card is also it's best one. The Black Lotus has been the king of the cards for years, initially printed in the first set of Magic: the Gathering. Black Lotus is an artifact that can be cast for free and allows the player to sacrifice it for three mana of any one color. The key behind the Black Lotus' power is that it gives players the ability to cast powerful spells much earlier than they usually would, and most importantly, it lets them do it as soon as the game begins.
Why MtG's Black Lotus Is So Sought-After
In card games, there is a small window called 'turn 0.' Turn 0 is the time when the first player is taking their first turn, and their opponent hasn't had their turn yet, leaving them defenseless. Black Lotus and similar cards like 'Moxen' let the first player take powerful actions during turn 0, leading to games that are over before they've even started. A classic example of this sort of play is the 'Channel Fireball' combo, which was so famous that one of the largest Magic: the Gathering retailers is called Channel Fireball. The combo involved playing a Black Lotus, sacrificing it for three green mana, casting Channel, and then paying 19 life to kill the opponent with the spell Fireball instantly.
Several months ago, there was a brief contender for Black Lotus' crown, Lurrus of the Dream-Den. Lurrus was so powerful that it had to be banned from the Vintage format until its core mechanic, companion, was nerfed. Lurrus was oppressive when it was around and was one of the most dominant cards of all time, but a lot of its power came from the fact that it allowed players to recast, and reuse cards like Black Lotus. In fact, in formats like Standard and Pioneer that didn't have Black Lotus or similarly powerful cards, Lurrus wasn't even banned. The cards around Lurrus were what made it so dominant, not Lurrus itself. Magic has been releasing more overpowered cards than underpowered cards recently, unlike Hearthstone, but Black Lotus and the rest of the 'Power 9' are on another level altogether.
Black Lotus is a relic of the past and is never going to show up in Magic: The Gathering's new e-sports push. It's only played in one sanctioned format, and Vintage is the format with the smallest player base, but there is a reason it's a household name. Magic: The Gathering is getting close to printing its 25,000th card, but none of them have been able to knock the Black Lotus off its throne.