Metagross is a potential defender as well as a powerful offender, we call the type as a 'tank'. so, to choose logically we have to see few of its characteristics:
- its main offense lies in its attack stats, main weakness is speed. it's weak on special defense.
- its preferred ability is clear body: so, its attack stat cant be lowered.
- make it hold a 'light clay', an item to increase the duration of the move, 'light screen'
strategy: we can make a metagross which can harm its worst foes handsomely, conceal its special defense weakness, use its STAB + high attack stat... or, at a cost of any 1 of this, raise its speed (still not much good) so that we can help this tank to escape from its battlefield (not a glorious thing to do, but if you are willing, teach it Agility)....
movesets with logic:
1. earthquake: to take down fire (supereffective + high Attack stat but no STAB) and fighting ( types (high Attack stat + fighting types dont fly except for only 1, so high damage is assured)
2. light screen: to protect it from special attacks and also its allies... (so, it makes it a team man)
3. meteor mash: high attact stat + STAB + okay accuracy (to increase its accuracy in cost of power go with iron head) + may further raise attack
4. zen headbutt: STAB + high Attack stat but lacks accuracy
so, its main weapons would be light screen and earthquake, and in supereffective cases, meteor mash. in an era of the Fairy intrusion, metagross with meteor mash/iron head is a gem.
other available options:
hammer arm (high power, high accuracy, no STAB), bullet punch (+1 priority, low power, STAB), explosion (high power, low HP move, but, this trick is a well known one and thus well coped up by many), agility (raises speed), iron defense (increases durability, not famous)...
cons of my chosen moveset: its speed stat is low, so, your foe may shift to a fighting type Pokemon with high speed (e.g. infernape) and hit you with brick break. its a point when your light screen will be a great flop. but, think once again, is it really useless?? guess not, you have your other buddies to take that offender on and keep your fingers crossed for the next time...