Eternal Witness

Regrowth on legs, and the legs are the entire design. The effect was old (returning any card from your graveyard to your hand had existed for years), but stapling it to a creature body changed what the recursion was worth. A spell that does this once is a spell; a creature that does it once on entry is a permanent you can return, recast, blink, sacrifice, and reanimate, which turns a single graveyard retrieval into an open-ended loop with the right enablers. That reusability is why the card became green's default recursion answer rather than a one-shot card-advantage spell: it folds into any toolbox that can bounce or recur it, and demands only a graveyard worth raiding. The 2/1 body is the cost the design pays for that flexibility, fragile enough that it dies to almost any removal and trades down in combat, which keeps the value in the trigger rather than in the creature surviving. The "you may" clause is a small mercy in the other direction: with a graveyard worth recurring you almost always will, but the optional wording lets you decline a return you do not want, leaving a key piece in the bin to feed a delve or flashback cost later. What it returns is left entirely open, and that openness is the point: a land after a wrath, a key spell, another copy of a value engine, whatever the board demands at the moment it resolves.