highest profit for [work*time*cost/gain] was buy selling houses, you need to spot the best deals and catch the best moments and players though so it is highly RNG based
then comes selling content
then comes crafting gear to hc raider on day one/two of release
then it really depends on your server

at least this is how it works on our

our guild made about 800 mil during heavensward period only by crafting and selling gear (pve or glam) so this includes tems from maps, gathering, and some mats/tomes farming.
a member made about 300 mil just form maps, he literally spam maps everyday, idk he finds them fun.
one made and empire by playing real estate on our server, he was giving away mil of gils to random people whenever he felt bored, do you know what it feels to have 2 retainers full of gil, he placed them on the extra retainers so he dont waste them on the first occasion, as you have to buy them from mog station every month to unlock
our guildmaster made enough gil to provide us more than half ward

you just really have to find what is trendy at the moment on your server and speculate what would be next so you prepare for the future