1st you put a picture of the Mind Flayers machine to kill eleven but ehh. 2. … usually I put stuff about all their backstories and reasons and skills etc but there isn’t much to compare, I’ll just do story interference.
The Demorgorgon(s) made up most of the story, I can’t quite clarify if they are the “best” but they did damage as was basically everything in season 1 with Will’s disappearance and the introduction of the upside-down and everything else. This means they are a plot villain and would be like one of those villains who created the entire plot(like Flash and Reverse Flash or The 100 and the Grounders) and that’s is something, and fits too.
The Mind Flayer is the main antagonist in season 2 with it’s not so robotic creation being the main antagonist of season 3. The Mind Flayer is ruthless/cold(Hah!) in a different way than Vecna or it’s façades or the Demogorgieksbris. It has nothing to say, it just kills etc. It was probably the main antagonist of the series as it seems to be in control of the Upside-Down where every villain came from (with the exception of the Soviets and the experimentees)
The Demo Dogs are a supporting antagonist in season 3. The Demo Dogs are
Billy is the supporting/secondary antagonist of season 3(probably I may of forgot). Billy was under control by the Mind Flayer so this should just be credit to the Mind Flayer but I won’t give it to him, Billy is mainly fear powered and induced and I can’t make up much with him but his undesicive(can’t find the word) desicion.
Brenner is a Secondary antagonist throughout all seasons. Brenner is inconsistently Henious and that’s all I can say
Vecna/Henry/One is the main antagonist of season 4. Vecna is good, the best? Debatable. Vecna is ruthless/cold for once again a different reason than anyone else, Vecna is ruthless because of his past and disability(or whatever you call it) and that makes up for something.
Personally I choose The Demogoron(s)
I believe the best would be either Vecna or the Mind Flayer
Demogorgon
Martin Brenner A.K.A Papa
Vecna
Mind Flyer
Billy 
Demodogs
