I think Zeke is obviously a better antagonistic force, but the parallels between Reiner and Eren as well as the twist of Reiner and Bertholds' reveal make Reiner my favorite.

I think Eren would be the best written character here since he goes from being a hero to a complete lunatic by the end of the story. Most "hero to villain" stories fail pretty badly so them nailing that with Eren makes him the best villain overall to me.