It depends on whether you're counting by length or body weight. If you're going by body weight, then I think it'd be the Mekong giant catfish (Pangasianodon gigas), they recently caught a 9' and 646 (or thereabouts) pound specimen in Thailand. If you're going by length, however, there are a couple of fish that are longer than that... I remember reading about a 12' eel being caught several weeks ago (I'll try to find the article so I can post up more info), and alligator gars get to about 10' long. Also, there's the Thai giant stingray which gets pretty large, I think around 7' in diameter that's confirmed... and there are reports of rays 18' in diameter!
EDIT: Oh yes, and the Piraiba catfish can reach lengths of 10 feet, as well.