It depends on if you prefer the taste of the fish itself or the sauce and accompaniments. One of the simplest and best things I ever ate at WDW was the cedar-planked Copper River salmon (sockeye salmon) at Artists Point, but it is available only for a few weeks per year in late Spring, I believe. I haven't had their "non-seasonal" version using Pacific king salmon (chinook salmon), but I'm sure it's almost as good.
As far as more "chef-driven" fish entrees, Flying Fish's catch of the day has never disappointed, no matter what it was.
I've actually had remarkably good luck with fish at WDW by steering clear of buffets and quick-service, and avoiding ordering fish at TS places unless it is one of their specialties.