Doesn't make you the most though. Fishing.. if you're good, yeh the more expensive fish can do well for you. Not the easiest though. Mini shops are a good way - mine has made me a fair amount of gold, and would do better if I was more active with it.
Buying and selling items in the exchange or marketplace is potentially the biggest earner. Buying donation items and selling at higher places can be a high earner if you invest in the right things.