Great topic!
Based on likability, I'll naturally go with As Nodt and Opie. Yes, they were quite villainous at times and they were capable of some really atrocious acts, but they were clearly trying to protect and please their master the whole time. It's almost impossible for me to hate someone when all they're doing is trying to be helpful but not realizing the amount of damage they cause. I consider both of them as well as many of the other Sternritters only half villainous for this reason.
As far as the ones I thought were truly villainous, Yhwach has to be at or near the top since not only does he act extremely villainous, but he corrupts others to do evil things as well. Aizen is pretty villainous as well, but he's not nearly as bad as Yhwach because he's not as manipulative towards his underlings. I would add Yamamoto for similar reasons, but he has shown some willingness to change and become a better leader.
I also agree with several other posters that Tsukishima was pretty villainous as well. He was very handsome and charming, but seeing him in Ichigo's house amongst his friends was a truly terrifying sight. I still consider Giselle quite villainous because she acts like such a dangerous predator. Also, I don't buy the fact that she turned good like Bazz-B and the others. She's one you can never fully trust.
