Adarkar Sentinel

Five mana buys a 3/3 with the most thankless activated ability the game's first years could put on a colorless body: pay a generic mana, get a single extra point of toughness, and watch it expire when the turn ends. The arithmetic is hostile to the card itself. Each point costs a full mana and refunds nothing, so a wall that absorbs one swing reverts to a plain 3/3 by your next combat unless you keep pouring mana into it. The activation has no ceiling and no payoff beyond not dying, which makes it a sink rather than a threat. What the card documents is how cautiously colorless creatures were priced before the type had any identity of its own: castable in any deck regardless of color, and taxed steeply for that universality. Hold it up against the keyword-laden, aggressively costed artifact bodies that came later and the distance between them is the whole history of how the category grew teeth. The toughness line does give it a floor, a blocker that can trade up against a lone attacker if you commit enough mana, but that floor is the entire pitch, and a defensive activation with no upside beyond surviving is precisely the design idea that did not survive its decade.