i don't think the mekong wins for weight, aren't there alligator gars that top 600lbs? ive seen a show called hunt for big fish were a guy caught a 400lb piraiba catfish. i don't think a 16' gigas would exactly be light either. and isn't there monsterous sized wels in the russian lake?
There are a couple of things i was wondering about:
1) the wels cat has been found in the black, baltic and aral seas so is strictly not freshwater.
2) although i dont think the mekong cat has ever been sighted in seawater if it is anything like many other members of Pangasiidae it could be migratory.
The white sturgeon, Acipenser transmontanus, sought by sportsmen on the Columbia and elsewhere along the West Coast, is the largest freshwater fish found in North America, attaining as much as 1,800 pounds and 20 feet in length.
I've got a fish book that says one type of sturgeon gets 29 feet long...I don't believe that though. I've never seen any pictures of arapaima larger than 5-6 feet, although I don't doubt that they get much larger than that.