I think debating which fish is the toughest is about as likely to change minds or be a productive use of one's time as debating cats vs. dogs or religion and politics.
But what the hell...? Why not? Nothing may get resolved -- but it's fun. Not as fun as actually catching the damned things, mind you, but if you can't be doing that, may as well be talking about it.
What brings this up is that Drew Townes, one of Sport Fishing's ad reps, a man so sharp he makes needles feel dull, asked several editors in a recent email, which are the strongest fish, pound for pound?
My top three (not in any particular order): giant trevally, swordfish and any of the large tunas (certainly including dogtooth). Of course most jacks could be right up there -- AJs, yellowtail (California or southern), samson fish, crevalle... hell, pound for pound, a blue runner or pompano gives a pretty account of itself!
I was surprised at some of the other answers -- just not species I'd have thought of, such as mako, peacock bass, smallmouth bass. Fun fish and acrobatic, to be sure but: toughest? I've caught 'em all. I don't see it but maybe that's just me.
One species I have not caught that is reputedly a beast to best is the "black bass" — a lower-river snapper (of the true snapper family, Lutjanidae) of New Guinea. Apparently hooking these 10- to 40-pound snapper is akin to snagging a passing locomotive, and that with their preferred snag-laden habitat, necessitates a locked-down-drag situation with very heavy braid. Of course all snapper, including cubera, are very powerful fish. But if taken in open water like tuna or swordies, I don't think they’d last so long.
Do you? I feel no shame in raising the ugly head yet again of this eternal debate among anglers. If it generates some heated debate, so much the better. The question will never be settled — but it will always be fun arguing the answers!