Battle Moveset

A Good Moveset for Metagross would have to be this:

Attacks:

Agility
Meteor Mash
Earthquake
Sludge Bomb

Items Attached:

Liechi Berry

Preferred Nature:

Adamant

Strategy Using Metagross

Metagross has all round stats, most are average or above average. Its speed isn't too great but with Agility this increases its speed.
Its ability Clear body prevent stat loss from abilities such as intimidate. Meteor Mash gains a boost from STAB and even though its not very accurate, it is one of the better steel type move available to Metagross. If you don't want to take a chance with this move, you may want to replace it with Shadow ball/Brick break.
The nature benefits Metagross already high attack but if you want to go for more speed than power, I suggest a speed benefiting nature and hindering its special attack(however, it is not needed with the move agility available to Metagross, but thats the matter of opinion)
Well, you know, Metagross is part steel type and that leaves him to very common weaknesses, fire types and Earthquake. Both of which are quite common and could easily beat Metagross.
EV's boost Metagross's Attack, special defense and defense, and with the boost in defense, it makes it slightly safer to use agility. If you come up against a pokemon you suspect has earthquake, or a fire type move then I suggest switching out.
Sludge bomb and Earthquake, both very useful moves and they have high attack strength Sludge bomb(90) and earthquake(100). Earthquake also covers its fire weakness. Overall, Metagross is a pokemon that can learn a wide range of attacks and with high attack, Metagross is one of the best steel pokemon out there, if not overused.

EV Corner:

Max out the EVs in Attack for Metagross with any remaining in Defense & Special Defense

Strategy Against Metagross

Metagross has High Defense and lower Special Defense, so your best bet here is to attack with Special Attacks or Super Effective attacks such as Fire, Ghost or Dark.

Contest Moveset

A good Contest Moveset for Metagross would have to be this for the Tough Contest best with Impish, Lax, Relaxed or Bold Nature:

Attacks:

Iron Defense
Double Edge
Rock Slide
Explosion

Items Attached:

Yellow Scarf

Strategy Using Metagross

If you do the attacks in this following order you should have very few problems; 1st - Iron defense, 2nd - Double edge, 3rd - Iron Defense, 4th - Double edge/Rock slide, 5th - Explosion

Locations in Games

Ruby/Sapphire Evolve Metang Colosseum Snatched From Cipher Nascour in Realgam Tower Fire Red/Leaf Green Trade from R/S/C

Animé Appearences

Metagross has had one Animé Appearence. It was in LaRousse City under the control of Hitomi. It helped defend the city from Deoxys but it did not do much else Movie 7: Visitor Of The Space Fissure!