These are the Memoria that increase the drops of event AP or event currency. Getting these Memoria and equipping them to your Magical Girls will make future farming more efficient, so naturally you should aim to buy these out before anything else. In addition, these Memoria are event-limited, and you may not get another chance to obtain them after the event ends.

If you don't intend to farm a particular event very much, it may be more efficient to only buy some of the available farming Memoria, or even to skip them completely, if you don't feel particularly completionist about them. In that case, farming Memoria can be moved significantly further down this tier list at your leisure. Do note that having a large stock of farming Memoria at hand can be helpful if the event is ever rerun, however.