It mostly depends on your server's economy.
1) Go to the AH, compile a list of popular and 'farming' items - such as raw ores, bars, herbs, gems, inscripts, pots, etc. and write down the prices of said items.
2) Research more about the certain items. E.g. where did the gems come from and how were they cut, what items are needed to mill certain items, etc.
3) Collect maps that provides the raw items. (e.g. Hellfire drops Fel Ore and Felweed)
4) Categorise the maps along with thier items (the items that you want to farm) into Groups (e.g. Hellfire = Group 1, Basin = Group 2, etc.)
5) Calucate the income gold of each group and find out which group has the best income... (don't forget to use the items' drop rates to calucate)
6) there you go.
It's recommended to have multiple characters with different professions (e.g. Char 1 = Mining / Herbing, Char 2 = JC / Inscription, etc.)