Wall of Roots

The clever trick here is that the mana never costs a card or a tap: it costs the wall a slice of its own toughness, and on a 0/5 with defender that toughness was never going to do anything offensive anyway. The -0/-1 counter is the entire balancing mechanism, and it is metered against a resource the card has in abundance. Five activations is the theoretical ceiling, but a defensive wall rarely needs all of them; the body that shrugs off most early burn is still a healthy blocker after three or four activations for green. The once-per-turn clause is the real fence, capping the acceleration at one extra mana per turn rather than letting you dump the whole life of the wall into a single explosive turn. What makes it a different animal from a tapping mana dork is that the mana arrives without tapping: the wall can block and still produce, so it never trades its defensive job for its ramp job. That decoupling of "block" from "produce mana" is the design idea that has kept it in green ramp shells for decades, long after the dorks it competes with have been printed faster and cheaper. It accelerates and walls in the same turn, and the only thing it ever spends is toughness it was hoarding for free.