The Power Nine
The Power Nine is a term that is used to describe what most consider to be the 9 most powerful cards in all of Magic history. Due to the extreme power of these cards, most are banned or restricted in tournament play. Not to mention that they are the most expensive cards you'll come across. All nine cards were rare cards printed only in the Alpha, Beta, and Unlimited sets, meaning they were only printed from late 1993 to early 1994.
This card in a nutshell, is the rarest card in the History of Magic. It ranges from $2,500 American dollars to even as high as $20,000. Needless to say this card is hands down the most expensive card you'll find.
Due to the power of this card, its is often banned or restricted in tournament settings. For Vintage Tournaments, it is Restricted to one per deck. For Legacy Tournaments, it is Banned. Meaning it cannot be used in any deck.
The Five Moxs
These five moxs all produce a similar effect. They are similar to the five Basic Lands in that they add one mana of a specific color to their owner's mana pool. In most situations the Mox cards are better than lands because a player can play as many of these as they desire per turn while a player can only play one land per turn. The Ruby, Pearl, and Emerald all sell for around $600. The Mox Jet sells for $650, and the Mox Sapphire sells for $900.
Much like the Black Lotus the Moxes are Banned in Legacy and Restricted to one in Vintage tournaments.
Ancestral Recall allows the player to use one card to draw three more at an extremely low mana cost. It originated as part of a set of five cards called "Boons", one of each color, which gave three of something. It remains to this day as one of the best card drawing cards in the history of Magic. It hits as low as $900 on the trade market making it one of the higher end cards. In a sort of homage to this card, Wizards printed a card in Time Spiral called Ancestral Vision. Much like its Alpha counterpart, Ancestral Vision has the draw three cards ability for one mana, however it comes with the suspend ability, requiring the player to wait for 4 turn to cast the spell. Like all the Power Nine, this card is banned in Legacy and restricted in Vintage.
Timetwister holds great power due to its ability to get a hand of fresh cards with the possibility of drawing cards that used to be in that player's graveyard. This card sells for an upwards of $500, making it the cheapest of all the Power Nine. Like all the Power Nine, this card is banned in Legacy and restricted in Vintage.
A straight forward card that has been replicated many times. The ability to take extra turns has been replicated set after set, yet Time Walk remains the cheapest and least harmful way to take an extra turn. It sells for the high price of $900. Again it is banned in Legacy and restricted in Vintage. A old version of the card use to say "Target player loses next turn." which was quickly changed before the release of the Alpha set due to the misconception that the player would lose the game next turn. If the card had been printed as such and the point of the card was that the opponent would lose the game next turn, it would've been quickly banned for being a overpowered card due to only have to wait 2 turns before you win the game.