I have to go with Hela because she was incredible to watch onscreen and was played wonderfully by Cate Blanchett, plus she was tremendously threatening and had lots of personality. At the same time, I thought Vulture and Ego were also terrific villains, and I only picked Hela because I found her especially good, not because I found anything wrong with Vulture or Ego.
Truthfully, all three of this year's MCU villains were very good, so you can easily make an argument for any of them. I think Marvel has been stepping up its game lately with its newer villains. Last year when I watched Captain America: Civil War, I thought that Zemo was a step in the right direction. This year, I thought that Ego in GOTG2 was another step in the right direction, and I thought that both Vulture and Hela were excellent.