Don't worry too much about which to desynth except if you want specific items. I would say either leatherworker/clothcraft( for field demi...itmes for gatherer/crafters), alchemy/smith/armor( battle craft demi) and fishing(dark matter and gil (rare items). Though, fishing is a waste I think but easiest to level. Overall, I think desynth is a waste of time and don't understand why the game forces a 3 choice on players. Only pain in the butt is field demi items.. since most players to lazy to find the easy way that square gave to get these items. Meaning the price still fairly high on auction house.