Deathrite Shaman

The term "one-mana planeswalker" got attached to this card almost immediately, and the comparison holds because of what it does on turn one and what it keeps doing every turn after. Drop it first, and it taps for mana off a graveyard land before either player has fully committed: a one-drop accelerant that needs no land of its own to feed, only a fetchland crack or a discarded card to point at. From there the same body pivots into a grindy late game, picking off graveyard-dependent strategies one card at a time while draining or gaining two life as the situation demands. The design crime is that none of these jobs trade off against each other. Most one-mana creatures are specialists; this one ramps, hates, and reaches across the whole back half of a game from a single 1/2 frame, and the fuel it spends is largely graveyard cards that are usually free to access.

That breadth is exactly why it became a rare creature to draw bans across competitive formats. It distorted what a one-mana investment was supposed to buy: a hatebear that ramps you, a mana dork that punishes the opponent, a body that was correct in fair midrange and combo and graveyard decks all at the same time. The hybrid pip kept it from being a color-commitment cost; the mana ability kept it from ever being a dead draw. Few one-drops have forced a format to ask whether a single creature can simply do too much.