Right now has to be Negredo, whilst he may not be (technically) the best player we have, I do think that in terms of attitude, effort and impact he stands out, particularly when you measure this against expectations when he arrived.
In the past, I have to go for Franny Lee, if only for his guts in starting a punch up with Norman Hunter. By the way, NH didn't seem to fight back much, was he behaving or scared?