Commander Sam Vimes is the emotional core of the Discworld series. Other story threads (the witches, Rincewind, etc.) are just as entertaining, but the City Watch stories are the ones that tug at the heart and have the most profound messages. The most recent Watch books - Night Watch and Thud! - are easily some of the best books I have read in recent years. Terry Pratchett just gets better and better the more he writes.