Voltaic Construct

Untapping is a deceptively quiet effect until you aim it at something that produces more than it costs to ready, and this 2/2 Golem is built to be the engine half of an artifact loop. The math defines it: each untap costs two mana, so a partner that taps for two breaks even and nets nothing, while a partner that taps for three or more turns the loop into a mana faucet. Pour two in, ready the producer, get three or more back, repeat until your mana is arbitrarily large; from there you need an outlet to convert the pile into a win. The body is no blank, but the activated ability carries the card entirely. The narrowness is its price. It scans the board as a row of metal creatures and offers to ready one of them, but only the metal ones: never a land, never a mana rock, only artifact creatures. That restriction is the line between this and a colorless engine that could untap anything, and it cuts both ways, since it can just as easily stand up an artifact attacker (or itself) to leave back a surprise blocker. Inert on its own, lethal the moment the right second piece, a tapper that overproduces, sits down beside it.