I read a magazine article several years ago that rated the top 10 hardest fighting fish based on a pound to pound ratio. If I remember correctly, Bluegill was 3rd. Smallmouth, carp, and stripers also made the list.
From personal experience I agree with ML and others about water conditions and having a saltwater background produces the hardest fighters. So I would say a striper is the hardest fighting fish I've caught. They have the saltwater vigor, and in early spring will practically fight to their demise.
