Deceiver Exarch

The untap clause is the entire reason this exists. Tapping an opponent's permanent at flash speed is a fine tempo trinket: a defensive blocker held back, a land tapped down on their upkeep, the sort of incidental disruption a 1/4 body backs up well. But "untap target permanent you control" is the mode that turned a value creature into half of a two-card kill. Enchant the Exarch with Splinter Twin, and the loop closes: the enchantment makes a hasty token copy, the token enters and untaps the original Exarch, the original taps to copy itself again, and you repeat until you have an arbitrarily large army. Kiki-Jiki, Mirror Breaker does the same work from the other end, copying a creature whose untap mode then untaps Kiki. Flash is what made the package detonate on the opponent's end step, dodging the sorcery-speed window where they could have answered the threat on their own turn. The flash also kept it honest outside the combo: a 1/4 wall that ambushes attackers or sits in front of held-up countermagic mana is real value even when you have no intention of going off, and the untap mode quietly refunds a land or mana rock on the way down. That dual identity (modal utility creature in one deck, combo piece in another) is what kept it relevant for years, and it is why the combo it enabled drew a Modern banning rather than the Exarch itself.