It all really comes down to how much you want to make, how fast, how much gil/time you're willing to invest, etc.

Fastest big gil: Spiritbonding. You'll want an i90 weapon for this if possible, and if you can join a full party spamming CM's first area then you'll be able to spiritbond at incredible speed. The results, as Mike mentioned, are down to what you're willing to gamble. The investment is notable - you either need to craft or buy a lot of gear because you'll want at least two sets, if not much more. You can spiritbond solo, too, with a chocobo, at places like Mor Dhona's Garlean camp.

Biggest fast gil: Maps, the non-specific retainer ventures and a huge, big whopping amount of luck.

Steady gil: Regular gardening (huge investment: a house) and crafting (huge investment: getting to 2-4 star recipes).

Cheapest gil: Gathering, easily. Find out what's in demand (don't just check the price, but also how often it sells). Gather it. For any high level stuff, there's that big initial investment in melding all your gear, which you may not be able to afford immediately. But even if you are only level 1, you can still go for...

Fastest low gil: Shards. Gather them and simultaneously send out retainers for them and you'll pick them up like crazy. I'm pretty sure that if you gear your retainers (they'll have to be Botanists or Miners to get shards) once they are around L20 then you'll already be able to get that 60 shards per 40 minutes deal.

Most assured gil: Dungeon running. You won't get loads, but you'll get decent amounts that will boost your coffers to at least a noticeable degree if you run dungeons often enough, and if you're into battling more than crafting/gathering, then this is the one for you.

Biggest long term saver: Level all your crafts + gatherers to 50 and never spend any notable gil on the market boards ever again unless you actually feel the cost is worth the time you'd otherwise have to invest in getting whatever it is you're buying.

I might be a little off with some of these descriptions and most of it was already mentioned, but yeah.