What makes Zemo special is that he’s not special at all. He’s just a man who’s experienced great tragedy, which he blames on the Avengers. “I have experience and patience. A man can do anything if he has those.”
Yet he succeeded in defeating the Avengers without throwing a single punch, causing them to turn on each other and making him the most successful villain in the MCU outside of Thanos. He simply wielded the weapons of fact and emotion to his advantage to divide and conquer. “A house divided cannot stand”, and that’s just as true for the people of the world now as it was for the Avengers.
Loki has the most screen time of any villain in the MCU, and this has given him an interesting opportunity to evolve. Thor was just as much his origin as it was Thor’s, and he stole the hearts of fans as the introverted and misunderstood little brother who doesn’t feel like he belongs.
But what makes him a truly unique villain is that he is deeply emotional. All he truly wants is to be appreciated for his gifts, even if that means playing the part of the “sinister villain” who wants to enslave mankind in Avengers. In a way, he actually succeeds in getting what he wants, as it takes the formation of Earth’s mightiest to defeat him.