ouch... that is almost impossible to answer, although I could make a list of candidates.. I wish i knew all freshwater fishes, but pike and eel would certainly feature as would stickleback. But if we allow eel and stickleback on the list, then that opens it up to my absolute favourite fish, American Shad (Alosa sapidissima). Not strictly speaking a freshwater fish (and nor are eel and stickleback fully)but for part of its lifecycle freshwater is an absolute requirement. As they are about to return here I will write an article about them soon.
in brief though, it belongs to the Atlantic Shad family and was absent in the Pacific Ocean until in the late 1800's they were transplanted to the West Coast and highly unusually have been phenomenally successful in establishig themselves without affecting "native" fish in any significant manner..although there are some grumblings from the salmon lobby but then in my view that has more to do with finding a scapegoat for their decline here other than fishing rather than anything else. The story, lifecycle and continued success of this fish on the West Coast and in the Pacific is an absolutely fascinating one, and I will go into some of that in at least one of my future hubs.
as for eating (previous poster), I haven't got a freshwater favourite, having discovered sablefish (or black cod as we call it here locally), smoked and then quickly heated back up in a wok for seconds.. pure heaven. Nothing comes even close.