Since leveling fishing doesn't rely on anything but time spent, it doesn't matter where you fish. However, if you're going to spend the time you may as well make some money.
I recommend the coast in Wetlands, many oily blackfish and firefin snapper schools. Also, the little lake in front of Stromgarde Keep in Arathi Highlands is full of mithril-headed trout, great for leveling your cooking or selling on AH for others to do the same. And if you don't mind dodging mobs, go down to Stranglethorn Vale and follow the river, best place to find greater sagefish schools, good money there as well.