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#1. Unstable Evolution
Expansion: Kobolds & Catacombs
Unstable Evolution used the Echo mechanic before it became a thing in the next expansion, The Witchwood. The most popular thing to happen (especially on Trolden) is getting a Sorcerer's Apprentice or Radiant Elemental from the evolution and then having 0 mana infinite Unstable Evolutions.
Unstable Evolution can get you many different minions of stronger stats and you can evolve your minions selectively based on the situation you're in, whether it's high rolling a Taunt or getting a bigger minion.
Honourable Mentions
Murmuring Elemental
Expansion: Kobolds & Catacombs
Murmuring Elemental was the first time the effect of a double Battlecry was seen since Brann Bronzebeard left Standard. It could be used to trigger your Elemental twice or if you reduced its cost to 1, you could combo it at 10 mana with Shudderwock.
Grumble, Worldshaker
Expansion: Kobolds & Catacombs
Grumble is an overstatted minion at 7/7 for 6 mana. Grumble saw a spike in Popularity when Shudderwock was released, bouncing back multiple 1 mana copies of Shudderwock.
