I love the unit responses of Defilers. They were all, "Hmm prah!" It sounds vaguely like they're saying emperor but it's all distorted. Defilers really represented the Zerg well. Plague was fantastic against Protoss just cause it went through their shields and brought the hit points down to 1.
Siaynoq's Playthroughs
Siaynoq's Playthroughs
Siaynoq's Playthroughs
For the Zerg, I really like the Hydralisks. Mainly because they're versatile and dependable. And for what they can accomplish, they don't cost much.
The Protoss, I'd go with the Dragoons. Besides aesthetic reasons, they do good damage for land units and can attack air units too.
Siaynoq's Playthroughs