Oooh I like the other answer that goes by colors. Ok, so here’s my picks for each color
Green
One of the single most broken creatures ever printed. It just is, abused with so many cards that can bring her back, and every time she can re-use any card you already used? awesome
Red
One red mana, put any creature from your hand into the battlefield. Red is actually probably the weakest color overall in Magic, but this effect is crazy powerful and I do think it is one of the more powerful cards in Magic
White
With one card, its a wrath of god effect + armaggeddon effect + discard effect. I mean, wow all with 2 mana
Blue
Blue has some of the most broken cards in the game, so it’s gonna be hard to pick just one. I’ll go with Mana Drain though.
Designers must have been high when they made this card. Now its even better due to no mana burn
Black
Also too many crazy powerful black cards. I’ll go with this one though
Re-use my black lotus and ancestral recalls? Why yes I will…
Land
Arguably the most broken land in the game. It’s either this or Bazaar of Baghdad or Library of Alexandria but I’ll go with the Academy
Artifact
Also, too many broken ones to name. I’ll go with this one though. I mean, cmon it may as well be part of the Power Nine, its arguably better than a Mox