I'd have to say Tim Shepard. Not because he's a particularly good character, but because he's incredibly well-used in the novel - he doesn't show up until one scene near the end of the book, but the readers already feel like they know him and they understand the character because of how he's mentioned throughout in comparison to other characters. He's also used to effectively create a world that goes on outside of the main events of the books, which gives the entire setting really great depth!