At the moment the most useful retainer to me is my DOW/DOM one. They can get chimera manes, amphithere skins, deepeye tears and other pain to gather materials. All retainers have their uses though so if you end up with 4 id get one of each honestly.

Also I looked you up and it says you dont have any of your DOL leveled, retainers cap out at your level on the job they are. So you'd have to level DOL if you want to use a DOL retainer of the same job. (DOW/DOM are the same way so youd probably want a conjurer retainer)