There are two questions to answer here. If you plan to farm for desynth fodder, where do you currently want to farm? And, what is your most commonly played class (or classes)?

The vast majority of Allagan gear (the i90 stuff) is ARM. The vast majority of the ST stuff is LTW. Dungeon gear depends on the dungeon, but is often a good mix of the standard classes for the gear. Remember that pants are always (or at least nearly always) one class "lighter" than the rest.

PLD, WAR, and DRG gear is very often ARM desynthable. We can reasonably assume that DRK gear will be as well. If you primarily main one of those classes, ARM is a good plan so you can roll Need on that gear.

BRD, NIN, and MNK gear are often LTW desynthable. Not all of their gear, as some of it is WVR and ARM, but a fair amount is LTW. We can reasonably assume that MCH will be in this category too.

WHM, BLM, SCH, SMN gear are very often WVR desynthable, but sometimes LTW instead (almost never ARM). We can reasonably assume that AST gear will be in this category too.


Of course, there are other factors to consider. LTW is the best class for getting FC3s right now by a long shot. ARM is the third best at getting MCDs (but you have the best (BSM) and second best (GSM), so whatever). And so on.