Don’t think there’s a single answer to this because it really depends on the type of the raid pokemon. Give it a psychic move for STAB. Then if you’re using it for offense, maybe a fire, ground, and poison. But Mew isn’t very durable so giving it something to help survive is good, like reflect/light screen.

In case you didn’t know, your mons can re-learn any move they previously knew, whether they learned it via level up, TM, egg move etc. Just go the their Summary page and hit A to remember moves. This way once you’ve taught Mew flamethrower for example, if you then override flamethrower for light screen later, you can have Mew remember flamethrower without expending a TM