Well, if I may express my humble opinion ( 

 ) , I want to see Brainiac because he is an intelligent foe for Superman, he is strong, has power, and good ressources (machine control, duplicating himself), and above all because he has good knowledge of the krytponian culture (I'm talking about the kryptonian Brainiac only) and that make him a great challenge for Superman. He knows how to hurt, using Superman's culture, family, heritage, homeworld. That is interesting, IMO.
But I like the Eradicator fior the same reasons. I often vote Brainiac or the Eradicator. On the contrary, I don't want to see stupid monster like Doomsday, in a movie.
If they go the kryptonian Brainiac, like they did in the cartoon, or more recently in Smallville, I'm all for it! But if they go the Vril Dox, green alien (well, he's cool too!) , I'd rather like to see the eradicator.