Depends entirely if you're farming triple gears, I'd say.
Bhaflau: Has only 2 ramparts you need to activate for the gears (and if I'm not mistaken, the highest ensured alex potential next to Arrapago). Boss is easy with simple HQ pop conditions. Slightly slower start with enemies having much higher HP, but Chain Affinity still wrecks flans.
Silver Seas: Has the advantage of ramparts instantly using Reinforcements (at the cost of them never dropping alex). Easiest HQ boss pop condition, just open the door before a certain amount of time (surprisingly plenty). Bosses are probably the most dangerous, though.
Zhayolm: Nude until 5th floor (barring lamp goodies). Most of the gear farming potential is on floor 4. HQ boss pop condition is not timed, just kill the chariots and gears on floor 6. Boss is easy too.