Again a question that is very hard to answer.
Size for size in the UK freshwaters, there is nothing to touch a fit well conditioned overwintered rainbow trout. After that I would say a sea trout.
Of fish in salt water, a mullet really takes some beating as well as a bass.
Other fish that really fight are male tench, barbel and common carp caught from rivers.
Brown trout come well down the scale here. I would put grayling as a better fighter that a brown trout.