I think Isam / Luc is the most interesting villain. You really don't learn his back-story until AMoL, but the fact that he hunts the hunters (wolves) both in TAR and in waking world with his ability to shift between them at will makes him so formidable.
From the first four books I would say the best (worst?) villain was Liandrin. Those Taraboner honey colored braids and stupid rosebud mouth...
Liandrin is spiteful and wicked. What she did to Egwenewas unforgivable. She is darkly cunning and clever, managing to always stay a step ahead of her pursuers. Breaking oaths, breaking women - it is hard to find anyone more villainous.turning her over to the Seanchan, which is especially terrible coming from another woman who can channel
Plus, she isn't even funny like Fain or Lanfear. She is just scary and bad!!!