Phyrexian Altar

Two functions fused into one rock: a sacrifice outlet that asks for no mana to fire, and a mana source that turns dying creatures into colored mana of any kind. That combination is what makes it an engine piece rather than a utility artifact. Ashnod's Altar got there first by producing colorless, but the color flexibility here is what unlocks recursion-and-rebuy loops: a creature that can be returned for any color, sacrificed back to the Altar, and recast on the same mana it generated. Pair it with a creature that comes back cheaply and a payoff that triggers on entry or death, and the Altar is the conversion step that makes the cycle profitable instead of merely repeatable. The card asks nothing of its controller except a creature to feed it, and that low cost of admission is what anchors it in so many notorious infinite-token and aristocrats loops: it is the colorless-to-colored bridge that lets a loop pay for itself. Producing mana of any color is the detail that keeps it from stalling on output the way colorless-only sacrifice engines do; it can fund the very spell that restarts the loop. A clean piece of combo plumbing whose elegance is that it does two jobs at once and demands almost nothing in return.