From a design perspective, I have two favorites for two different reasons. It's a total coincidence that they're both Zerg units. Please don't read too much into it!
I wasn't on the team during the initial development of WoL, but the Baneling must have been a very controversial unit. In general, I think there's this idea that people don't like using suicide units; it just doesn't follow the common RTS fantasy of building up the most powerful army possible over time. (For similar reasons, Zagara was thought to be a very risky Co-op Commander.) And yes, I know Scourge were a thing in SC1, but the Baneling was designed to be a core unit in SC2, something players are supposed to interact with in nearly every game.
Though there's certainly some controversy about this unit, especially recently, I think overall, this unit turned out beautifully and certainly exceeded my expectations. After all, it's a key component of one of the most beloved interactions...
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