There are two that stick out in my mind. First is of course the Castillo fight. The second one might seem a bit weird but its his fight with Floyd Mayweather and his tremendous demonstration of heart by climbing off of the canvas 5 times, despite the hopeless situation. Many fighters would have stayed down knowing the outcome was already decided, but Chico showed the heart of a warrior and a true champion by continuing to get up.
RIP and much respect.