If we're talking strictly in Part 1, then I would say either Itachi (sheer power-wise) or Orochimaru (no one is more sinister than he is)
I don't think Kimimaro qualifies because he wasn't really a "villain". He was only trying to prove himself to Orochimaru-sama because he didn't want to be alone. He was exactly the same as Haku (minus the fact that Haku was clearly gay)
itachi- made jonins look like scared children, introduced the MS and its powers, destroyed one of the strongest characters at the time, and escaped jiriaya inescapable jutsu. I'd say itachi is the best