I suspect this one will get old fast, but since I'm one of the first to post in this thread....
I find Spike fascinating. And no, it's not just the cheekbones or the abs, he really is one of the most interesting characters on Buffy. Right from the beginning we're set up with this completely simple view of vampires and evil. They lose their souls when they're turned, they become evil, and the Slayer must kill them. And of course, Spike is introduced to us following that exact same mold; he was a murderer, he wanted to kill Buffy, and Buffy therefore tried to kill him. Yet, at the same time he was always different in his treatment of Drusilla; he was incredibly loving for a monster who couldn't love. And for all his evilness, he was always... likable, and human, too. He could cry on Willow's shoulder and then threaten her with a broken bottle within the same scene, and none of it ever seemed out-of-character.
Then, of course, he gets the chip and falls in love with Buffy. While some Spike fans find that this ruined him, I actually enjoyed the development. He showed both a scary, obsessive, making-robots-of-her love, and a selfless, giving, I'd-die-for-her love, and all in the same episode!
He has nothing in him that says he should be good, and yet he does things that are innately right. His need to comfort Dawn (I always loved the scene when he reached for her, only to pull back when she turned his way), his willingness to walk into battle for Buffy and his increasingly changing personality always kept my interest. Here was a character who went against every single rule that Joss and Co. had built in the Buffy world, and I was captivated by that storyline.
Of course, I had a lot of complaints in subsequent seasons, but his journey always held my interest.