It is very close between Chandulure and Bisharp, but I'd say Chandulure
I like Chandulure for having such an original design, secondly it has what a ghost type should have; a creepy origin. Chandulures are said to creep on other creatures, then suck their souls out and then BURN THE BODY INTO ASHES.
Also I like Chandulre, because of its battle capabilities, as it has the highest special attack of all non legendaries.
Lastly, Chandulure was part of one of my signifcant moments in Pokemon history. I had two pokemon against three when I was battling Ghetsis. I had Chandulure and Emboar, while Ghetsis had Elecktross, Bisharp and Boufallent. Emboar was very low on HP, but through a stunning series of events my lone Chandulure won the final battle, which is why Chandulure is my favourite Gen 5 pokemon