Throughout the entire MCU there has been a real mixed bag of villains. Towards the beginning of the franchise, the bad guys that our heroes faced were not exactly the most compelling of characters. Obidiah Stane was a good villain, but was followed by Abomination, Whiplash, and Laufey. This theme continued in Phase Two, with forgettable antagonists like Ronan the Accuser and Malekith appearing in Guardians of the Galaxy and Thor: The Dark World respectively.
Part of the problem is that there aren't many heroes that have appeared in only one movie, but still, there aren't many villains who have appeared in more than one movie. So much time is dedicated to the good guys, the villains often feel under-developed.
Phase Three, for the most part, was different. The villains suddenly became complex characters with real motivations that the audience could empathise with, and actually posed a real threat to the heroes. Instead of antagonists devoid of any personality, Phase Three became a lot more personal with its villains.
Of course, not every single one was a home run, but with the exception of Loki, the very best of the MCU's bad guys come from Phase Three.
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