Its a toss up between:
Xander, the human with no powers or special training (besides the slight residuals from the halloween episode that turned him into a soldier) that always fought alongside his friends, despite clearly being the weakest for much of that time. He also provided a lot of the funny.
Spike, the Slayer slaying vampire, a souless human possessed by a demon, who fell in love and went through hell to get his soul back, only to not get the woman he loved, and yet stayed good. All hile being hot, and "British".
Of course I love Spike's story in Angel too, whilst Xander's ended in Buffy, so Spike has a potentially unfair advantage.
Who's yours?