The fastest method would be barb fly fishing using the heavy rod. You have to bring your own bait and make no proit whatsoever. This is because you have to drop the fish to get the fastest exp. I did this to get 99 fishing. Alternatively you can fish monkfish if you complete Swan Song, a quest. This makes you some decent cash and a reasonable exp rate. Sharks, tuna, swordfish all return bad exp rates although can bank them and earn some money on the side.