It was hard to pick one, the best for me were Buffy, Willow and Spike, with Anya a bit behind them. Out of all of them Buffy is the only one who always remained true to who she was, she never let herself drown in the darkness around her or allowed it to take her over, even during season 6 that was emotionaly a terrible one for her and in season 7 she was betrayed and abandoned by everyone in her life, including family and friends, except for Spike, so I guess I'll go with her for that difference, because all of them went through a lot and had a great arc
