If it's the masterminds, then I must say Kageroza and/or Muramasa.
They gave birth to an amazing span of interesting episodes with fights XP
If it's generic.. Meh, I don't really have one in particular. Then again, I could have forgotten some. Findor and Halibel are close though
because they have WATER POWER! DESPAIR!!! MWAHAHAHAHAHA
Nijigasumi is also an interesting zanpakto.
Buuuuut still. That's about the power alone. For the concept (i.e how the villain thinks/plans, how he/she carries out his/her plans, etc) we don't really have much choice. I mean, so far we have, Aizen and Tsukishima as the main villains; the others are simply pawns. (Maybe Jin Kariya if you consider the Bounts, and the once-noble-families-of-sereitei in Movie 1 whom I cannot remember, and Kusaka, and Homura and Shizuku, but I won't really place them here.) So.. as we have pointed out, Aizen is just too perfect
to be a likeable villain, while Tsukishima, so far, shows no "qualities" of being a good villain. (Good in the sense that he has good plans, moves etc, not good as in literally good. XD)