Okay this one might be a little difficult. I even find myself liking all the villains I write. But I have to pick so I'll go with Velup the Sorcerer.
Velup is the prime villain in Uritus and the Sword of Fire. I do like him a bit because I created him, but he's so evil and is the prime cause of all my protagonist's pain so I have to pick him. Velup is the traveling Sorcerer, and brother of Uriah the Wizard (who is good). At a young age he went mad and sett his own house on fire with his parents in it, causing him to die. He then turned to sorcery and sought out an apprentice - Ercildus, Uritus's big brother.
Basically, he's a jerk. I celebrated at his defeat. And so that is why he is my least favorite character I've written.