Spike, duh ! He's my favorite character (beside Buffy).
He's not perfect. He assumes who he is, the bad as much as the good, and doesn't care about what the others think about him. He does things in his own way, he listens his heart, and most of the time, he learns from his mistakes. I love his emotional insecurities, when we see the real Spike who usually tries to hide himself behind sarcasm. He also looks for evolution, that's a beautiful quality, even if he fails sometimes, he tries and he keeps trying. That's the most important part to me. And I simply love who he is and how he handles things. Even if he perfectly can be a jerk sometimes. That's a part of his charm 😛 His character has an amazing journey and I think he's one of the most interesting and complex character, in particular during seasons 5 & 6, when he's looking for an identity and is lost between this duality good/bad.
I like Angel, but most of the time, I feel pretty much indifferent to his character.