Spirit en-Kor

Zero mana, repeatable, and that is the whole trick. The redirect clause reads like a smoothing tool: route the next single point of incoming damage onto a creature you control rather than absorbing it, splitting a burn spell or a combat hit across your board piece by piece, or shunting it onto something that does not mind eating it. But the rate hides the detail that matters: every free activation has to target a creature you control. That targeting is what the famous en-Kor loops exploit. Pair the spirit with a creature whose ability triggers on being targeted (Cephalid Illusionist mills its controller, Daru Spiritualist pumps a toughness) and the zero-cost ability becomes an unbounded targeting engine, the redirected damage incidental to the loop rather than the reason for it.

The structural lesson is precise, and it is one Wizards keeps relearning: it is not the redirect that breaks the card, it is that a free, repeatable ability must point at something, and free repeatable targeting enables combos no matter what the rest of the text claims to do. The cost and the effect are separate problems, and this card costs nothing. The flying 2/2 body is the evasive frame for the turns when no loop is assembled, a perfectly fair creature wrapped around an ability that stops being fair the moment the right partner sits across the battlefield.