Oh my oh my. Now, that's something I can talk about. You see, no matter what show, game, book or whatever else I get into, villains end up being on top of my favorite characters list, and honestly, I dont know why I never asked this question myself.
Now, speaking of Reborn, I can't really pick just one character. You see, first of all, we've got Fern. Yes, I'm one of those few people who actually like this guy. He didn't really started as a villain, but we sure can consider him as one now. I can't really explain why I like him so much, but apparently he fits that one certain type of character I'm weak for: egocentric jerk with unbelievably high, even too high, self-esteem. There's also the fact, that he's not completly rotten inside, since in E15 we can see how much Florinias attitude towards him (and pretty much everything and everyone else) really bothers him. That being said, let me sit here and wait patiently for Fern's redemption arc.
Next, we've got Doctor Sigmunt Connal. Now, theres something Ama mentioned a couple posts above, explaining why he sympathises with Solaris, and thats the exact same reason why I like Connal so much. Even if his actions are more than questionable, he genuinely believes that what he does is the right thing to do. Still, some research is necessary to find out his reasons, since it's not mentioned in the main plot. Also, he somehow reminds me of Kotomine Kirei from Fate series and don't even make me star talking about how much I love this guy.
Finaly, ther's Blake. Not sure if it's intentional or not, but this guy shows so many psychopathic traits that, as someone who's studying forensic psychology, I simply can't not be interested in him. Seriously, analyzing his behavior is just too much fun. Plus, his lines can really be hilarious sometimes. "I've got things to do, people to meet, worlds to end. You know, typical Sunday"
I should also mention Taka here, but to be honest, I can't really see him as a villain.