My all time favorite X-Villain (and favorite X-Character in general) is Mastermind. The original, not the two that came after him that were apparently new and improved because they were hot chicks.
Reasons for liking the original mutant master of illusions are simple, but I still get odd looks when I tell people that he's my favorite. I like him because he's smart. His power is simple, it won't blow holes through mountains or kill you in five seconds. He's admittidly scrawny and weak in physical power and he's not going to be wining any mutant beauty contests....
...But he's smart. What I love about him is that he'll set things up in such a way that unless he wants you to know what's going on, you ten to one won't. He's a wonderful example of how a relativly low powered mutant can be a deadly threat. If written correctly, Mastermind could concievably defeat the X-Men without the aid of any other villains simply because he knows the best way to do things is with long time planning and deviding the power of your opponet.
So yeah. Glad to be here and hope to have many good discussions.