There are many ways for players to hoard riches in Final Fantasy XIV, with most methods centering around selling items on the market boards. While market prices can fluctuate, there's a way to keep the gil flowing while waiting for your next hot item to sell — Levequests.
The market board's activity and prices can fluctuate each day, and your "guaranteed" money-maker may take longer to turn a profit for you. The gil reward of a singular Levequest may not make the same impact on your funds as selling a rare minion, but using your daily allowances on them can keep a steady stream of gil flowing over time.

Players gain three Levequest allowances every 12 real-world hours, totaling six daily. By incorporating your Levequest allowances into your FFXIV schedule, your daily routine will have an additional chunk of funds with a trivial amount of work added. With achievements that require earning upwards of 10 million gil from Levequests, it'd be a good idea to fit your allowances into your schedule.
However, not all Leves are equal, as many early Levequests introduced back in A Realm Reborn don't have the same yields as the higher-level quests introduced later in Shadowbringers. Some Levequests might have a large gil reward but require far too many steps to be an optimal money-making strategy. Knowing which Levequests to look for while make the most optimal use of your time.
The best Levequests for earning gil in FFXIV
Historically, the best money-making Levequests are Tradecraft Leves that only a Disciple of the Hand job can complete. These quests typically have you craft any number of items and turn them in for a reward, doubling if the item you hand over is high quality.
You'll need to know which Levequests are the quickest and most efficient uses of your time and allowances. This guide will assume you have your crafting jobs at max level and have access to each Levemete.

"Practical Command" for Level 86 Alchemist
One of the current best Levequests for earning gil is the Endwalker Levequest for Alchemists, "Practical Command." Its completion requirements are to turn in three Commanding Craftsman's Draught, and its reward is a combination of crafting materials used in the item and approximately 10,000 gil if your turn-in is high quality.
The recipe that conveniently crafts three Commanding Craftsman's Draught at a time has no pre-craft requirements, meaning there are fewer steps to take between grabbing the crafting materials and turning the result in for your reward.
The recipe for three Commanding Craftsman's Draught requires:
The one hurdle you may come across when trying to use your time efficiently is acquiring Petalouda Scales. While you can gather the other three ingredients using Disciples of the Land jobs, Petalouda Scales are obtained through Retainer Ventures, defeating specific mobs, and exchanging Bicolor Gemstones. Thankfully, they're market tradeable, so you can gather them yourself or buy them from the market at a price where you'd still make a profit.

"A Stickler for Carrots" for Level 84 Culinarian
Competing for the title of "best" Levequest for earning gil is "A Stickler for Carrots," the level 84 Culinarian Levequest introduced in Endwalker. This Tradecraft Leve requires you to turn in three Carrot Nibbles, giving you a slightly lesser gil reward than Practical Command but still averaging around 10,000 gil for high quality turn-ins.
One strike against A Stickler for Carrots is that Carrot Nibbles require a pre-crafted item, Night Vinegar, adding to the time you'll spend crafting before turning in the final craft. The bright side is that each ingredient required for Carrot Nibbles, even for its pre-craft, is gatherable from your run-of-the-mill gathering nodes.
The recipe for three Carrot Nibbles calls for:
Night Vinegar:

At approximately 10,000 gil per high quality turn-in and six allowances, you can quickly earn around 60,000 gil per day if you spend a small amount of time gathering your materials to prep. Using your allowances daily could net you upwards of 1.5 million or more after 30 days if you're self-sufficient. If you've let your daily Levequest allowances stockpile, crafting and turning in either Levequest's requirement in bulk allows you to earn more of that total at once rather than over time.
Both options have pros and cons but don't differ enough for one to be objectively superior. Neither requires any materials from time-locked gathering nodes. Since they reward a similar amount of gil, it's up to the player to decide which method best uses their time and which sacrifice to make — spend the extra time gathering the Petalouda Scales for the Alchemist recipe or spend the extra time crafting the Night Vinegar instead. The third option would be to buy them from the market board if you aren't pinching pennies, which would obviously lower your overall profits but eliminate any time spent gathering.