Meekstone

A single number does all the work here, and the choice of three power draws a clean line in the sand: small creatures (Savannah Lions, Order of Leitbur, Mishra's Factory) stay under it, while the fatties of the format (Shivan Dragon, Sengir Vampire) stay tapped the moment they swing. A white-weenie or artifact-aggro deck could build its way below the line by construction; the opposing midrange and control decks leaning on expensive threats could not, and a Shivan Dragon that attacked once would not stand back up on its own. Vigilance creatures dodge the lock entirely (Serra Angel never taps to attack, which is why it was famously paired with this), and the printed wording is similarly precise: it only stops a creature from untapping during its controller's untap step, so a separate untap effect can still free it. This is a museum piece of pre-modern design discipline in two ways at once. First, the threshold is a static, printed number rather than a scaling or conditional one, which is how early Magic handled "asymmetric but fair": draw the line and let deckbuilders pick a side. Second, the cost is a single generic mana for a board-wide tax, with no activation, no upkeep, no off-switch even for its controller. Modern design would price that higher, gate it behind a triggered ability, or staple on a drawback; this just sits there and does it. Creature stat inflation has eroded the lock over time, but the threshold-tax remains a remarkably pure rendering of the archetype.