Power Nine
The Power Nine are the most powerful (and therefore expensive) cards in Magic: The Gathering, all first printed in Limited Edition Alpha, the original form of the game. Every single one of them is so broken that they are banned in every format except Vintage (a format that exists as an asylum for cards too powerful for the other formats) and even then they're restricted to one per deck.
The Cards
Ancestral Recall
Target player draws 3 cards.
That's three cards for one mana. If you're not immediately jizzing your pants at the thought of what you can do with that, you'll fit right in with the rest of /tg/ when it comes to Magic.
Time Walk
Take an extra turn after this one.
There have been many ways to take an extra turn in Magic, but this is and will always be the cheapest and least restrictive way to do it. This makes it trivial to set up a combo that lets you play the same Time Walk over and over again for infinite turns. In fact, for a while the sheer infamy of this card led to the DCI banning all cards of the form "take an extra turn after this one" from non-Vintage formats because of the distorting effect of infinite-turns combos, and all the extra-turn cards Wizards prints of late generally either have punishing preconditions or make you lose the game after the end of the extra turn.
Timetwister
Each player shuffles his or her hand and graveyard into his or her library, then draws seven cards.
Timetwister isn't as obviously broken as the other members of the Power Nine, since it's seemingly symmetrical, but if there's a large asymmetry in players' current hand sizes the casting player can get a downright degenerate level of card swing out of one card and three mana, while kicking any combo pieces out of the opponents' hands.
Black Lotus
Sacrifice Black Lotus: Add three mana of any one color to your mana pool.
Jesus Christ, how horrifying. Wizards has a long history of trying to "fix" this card, and every time except for one (Lotus Bloom, which takes three turns to get you mana and still ended up being the highlight of at least one World Championship deck) the "balanced" Lotus clone was still so hideously broken that it ended up getting banned.
The Moxen
Five artifacts, each imitating a basic land.
This is a big deal because while basic lands are restricted to one per turn to keep endgame from happening by turn three, artifacts have no such restriction; you can play them all at once to get your heavy hitters on the board while your opponent is still playing with two-drops. This was even worse when the Moxen were originally printed, because at that time there was no restriction on the number of duplicate cards in your deck. You could replace all your basic lands with Moxen with no downside other than the time and money required to get the cards.
Sol Ring
{T}: Add {2} to your mana pool.
Sol Ring is a ridiculous mana source from Alpha that is restricted in Vintage, which leads many to refer to it as the tenth member of the Power Nine, although Library of Alexandria is also a strong contender for the title. Unlike the "real" Power Nine, Sol Ring is weak enough to be considered legal in Commander, where the increased life totals and singleton limit on cards keep the ramp it provides from being too big of a deal.
The 10th piece of power
There has been much arguing over the years over the 10th piece of the power nine, the arguement has usually comedown to Library of Alexandria, and Sol Ring (which is played in almost every EDH deck and considered commanders signature ramp artifact). Obviously its -Shut up Newfag! Alright listen hear little one, anyone who tells you the 10th piece of power is either of those is talking you bullshit. The REAL 10th piece of power and (secretly) the best card in the game, which is literrally banned in every format due to it being Literally illegal to play in parts of America is...
Contract from below
For context, Richard Garfield himself introduced a mechanic into the game called Ante back when it started up. This worked that if both players agreed to play ante, after shuffling up, they would put the top card of their deck into the ante zone. Whoever won the game Legally gained Ownership of the cards in the ante zone, basically playing to own a random card of an opponents and keep one of your own. This caused a lot of controversy due to it incorperating gambling into the game, so wizards acted like ante cards didnt exist and procedurally banned them from sanctioned play. Funny that the strongest card in the game is an ante card. Also every ante card has the clause of exiling it at the beggining of the game if you arent playing for ante. But what is Contract from below? The literal deal with the devil card. Discard your hand, ante the top card of your library, then draw 7 cards. 7 cards. A whole new hand, one black mana. This is literally a one sided wheel of fortune for a single black mana, who cares if you ante a card? You are likely not losing if you just drew a new hand! This card would still be banned even if it was not an ante card. But how can I play with ante without ante? Thats simple! If you want to play with ante without ante, treat ante as exiling the card. So exile the top card of your library then draw 7 cards FOR ONE BLACK MANA. This is the best card in magic, better than black lotus.