Yavimaya Hollow

Land that protects creatures was a strange idea when this arrived, and it remains a quietly elegant piece of design. The Urza's-block cycle of legendary lands each grafted a colored utility ability onto a colorless mana source, and this one took regeneration off the spell slot entirely. That reframing is the whole point: a creature's insurance policy no longer occupies a card in hand, so the green deck holds its threat together without spending a separate trick to do it. The cost is paid where it hurts least, on a permanent that taps for mana when nothing is dying. Regeneration's well-known holes still apply (it does nothing against exile, against bounce or sacrifice, against the era's destroy-without-regeneration wraths like Wrath of God and Damnation that name the loophole shut), but against ordinary targeted removal and combat it turned a key creature into something an opponent had to overcommit to kill. The land also dodges the timing problem that haunts protective tricks: there is no window where you reveal your hand, because the ability sits open on the battlefield, ready at instant speed for a single green mana whenever combat or a burn spell forces the issue. A green deck's most important creature became, in effect, regeneration-on-demand without spending a card to get there.