The "Best" ways to make gil (legitimately) consist of two things.. i'll list them.
1. Become very good at the game. Find 4 to 5 other people who are equally good at the game. Be able to low man every delve and battlefield in FFXI. Once you are good enough at that, sell the battlefield/delve wins and gear for extremely high prices to the people who aren't as good at this game. (or are just lazy/rich)
2. Do the exact same thing as above but do it with alts and 4 or 5 box. This cuts out 4 to 5 other people and you can keep all of the gil for yourself.
These are by far the BEST ways to make gil. The ways people have listed here are fine. You will make gil doing them but it won't even come close to how much you can make if you are able to reliably merc wins/gear on a weekly basis.
Examples (somewhat hypothetical):
When you can sell Ouryu v2 drops for 10-30m depending on the item, you line up a bunch of buyers outside of the battlefield, clear it, and let them join the party to lot their gear after they pay you.
When you can low man delve, you invite the extra person/s to sit in the party and stay at the entrance. They don’t do a thing. You sell a win for 6-12m depending on the zone. You sell boss drops for 1-2m depending on what you can negotiate. You get EXP/CP/plasm for yourself + the boss material.
While you can make money in other ways (farming, sparks, accolades, ect) – Nothing is more efficient than mercing. There is just no better way.
This is how you make a couple hundred mil in a week.