I've been leveling up CRP and ALC. From what I've seen, those are probably not the best to level since they cannot desynth any gear but weapons and housing items. I will level LTW up shortly just so that it will be easier to get the higher level demimateria. GSM seems like it would be the easiest since jewelry is about 50 percent of your gear and 50 percent of drops in a dungeon.

The field craft and battlecraft come from their respective gear sets much like materia does with spirit bonding from what I've seen. Housing and fish bring in field craft as well.

I can't confirm this but it looks like anything that is dropped from regular dungeons is desynthable. Also CT. I can't confirm ST though since I've only collected Allegan tomes so far. And need to check on coil gear as well.