I dont really know what the hardest fighting fish is.. I think one thing that alot of people seem to have over looked on here, is the gear we use to catch them..
Alot of barbel anglers use I would imagine, something like 1.75lb TC, or something similar, where as carp anglers use 2, 2.5, going up to 3.5lb TC rods.. Chub anglers might use a float rod, perch anglers might use a very soft quiver tip..
I know that a perch is no match for a carp, but perch fight hard on light gear, as do chub.. Im sure if you played most barbel on 2.75 rods, they would comein a little easier than if you did it on 1.75 rods..
pound for pound is a good measurement to go by when judging fish, but I think the gear has alot to do with it too..
Although Ive not masses of experiance, n Ive not caught masses of carp, and have never caught a barbel or catfish or anything like that.. I have caught plenty of big perch, nearly 2 dozen over 2lbs, and 3 differant 3lb'ers all on soft quiver tips, with 4lb line straight through, and they all faught hard.. as have all the chub Ive caught, again a soft float rod with 4lb line..
Theres alot of strong fish out there.. I dont know which fights the strongest, but I know which ones I enjoy catching..