Probably Hightower, maybe Redwyne.
Sorry, misread. It is late.
GRRM says it’s the Freys in ASOS, but I think he made a worldbuilding mistake in AGOT if that is the case.
Should it have been Mallister? The house that’s supposed to be able to hold off the Ironborn on its own?
Them or the Bracken/Blackwood duo that can’t be held down by the Tullys of Riverrun.
EDIT: Or Harrenhal for that matter.
To be fair, Harrenhal is probably second most powerful, but as of ASOS there was effectively no Tully Vassal in command of Harrenhal.
My problem with Harrenhal is AGOT-related. Where did Harrenhal’s forces go?
Tywin’s easy capture of Harrenhal made it sound like they just didn’t have any of note, at least not to compare to the thousands of men all of the other major houses are throwing around. Possible artifact of Martin enlarging the setting late in his world building?
Harrenhal cannot simultaneously be one of the richest fiefdoms in Westeros – as it is described in AGOT – and not have armies of note. That’s not how feudalism works.