There are certainly a lot of drops here lol. But here's a simple way to remember most of the important ones.
Pick up anything "dirty" and if you want to make fast zulie, sell them as dirty too.
Pick up all refines and powders (essense. ether etc)
Check the shop of the NPC named Kiroth. He is located at the back of the city of Junon, near the guy who takes you to Kenji. Basically anything he sells, it's not worth picking up because any crafter can just go buy those items themselves.
Notice that each item has an quality grade. Kiroth sells up to qual 34 so anything higher than that is only from monster drops and can probably be sold to other players.
Every drop can be sold to an NPC so it all has value but because there are so so many drops, you can't keep them all even if you want to
As you go to the other planets, drops specific to them would always be something you can sell since a lot of people don't spend much time on them.
Hope this helps a bit