Nearly every metagross in existence uses the moves Earthquake and meteor mash, and for good reason, they cover a wide range of types. As for the other 2 attacks, I would suggest these 2 if you want " a fast one.":
Earthquake-Powerful, covers fire weakness
Meteor Mash-STAB, powerful move, okay accuracy
Agility-gain speed boosts once after a while in the battle when your opponents pokemon have been weakened, a well played metagross will usually have more health at this point, leading for sweeping since your metagross is fast, it should be able to outspeed most with a boost from agility, the only thing to watch for then is trick room.
your last attack varies from what your team needs. Many people will add explosion as a last resort for when metagross' health is low, many can see this coming and usually switch in with a ghost type.
Use a move that will compliment your team, covering any types that metagross can't hit with the previous moves mentioned.