Of all the animes you have watched, who was your favorite antagonist? For what reasons did you "love to hate" this character? Personally, my favorite was Archer from Fate Zero; his character was executed flawlessly, and I typically like powerful enemies as they make the anime that much more interesting.
I guess it depends on what kind of bad guy you're talking about, since some bad guys become aligned with the protagonist later in the story, etc.
I always liked Viral from Gurren Lagaan for some reason though. I don't want to spoil too much, but I think he has pretty good dialogue and characterization, and his character development through the series is pretty neat too.
Raito Yagami from Death Note.
I loved it how he went from being a good guy to being a bad one. That's just what power does to you, I guess. From killing only bad people to killing everyone who stood in his way, showing no mercy to anyone, not even his family members. There might be people more evil than Raito, but he's definitely my favorite.
He's portrayed as a bad guy in comparison to most of the other Servants as Fate Zero mainly focused on Saber as being the protagonist of the Servants, in my opinion. However, roughly an equal amount of time was spent on each Servant/Master, so it was up to the audience to decide which anime character to root for. In that case, I agree with you; the "bad guy" was according to the individual's perspective.