Well, I am truly humbled, thought I had been around and done a bit of fishing over the years; now I am embarrassed, haven't even got started.
Size is the problem as it is hard to compare a 20lb carp with gudgeon so, sticking with fish of around 3-5lb of which I have caught a few it still has to be that sea trout and the mullet second. That criteria excludes roach and who knows what a 3lb gudgeon would fight like. Never caught a salmon, tried and failed but, I imagine it would be top for British fish. But, that sea trout I caught really did appear to be "intelligent" whereas all other fish I catch just react in different ways so for that it gets a lot of kudos for me.
Very interesting replies. enjoyable.