Battle Moveset

A Good Moveset for Metagross would have to be this:

Attacks:

Meteor Mash
Earthquake
Agility
Explosion

Items Attached:

Leftovers OR
Lum Berry

Preferred Nature:

Adamant

Strategy Using Metagross

The standard Agil-Gross set. With Agility and a few Speed EVs it can outrun almost any un-boosted opponent and with it's huge Attack stat and reasonable Defensive stats it can destroy teams. With it's Great typing and move-pool it's a staple in player's teams for a reason. The usual strategy when using Agil-Gross is to use it as a Wall and an Attacker until all it's main counters are out of the way (Skarmory and bulky Water types) and then sweep with MM and EQ, using Explosion if you need to (such as knowing you can win the match if a certain Pokémon is out of the way) or if you're close to fainting.

With a resistance to Flying and Normal attacks it can make a good wall against opposing Choice Banders and with a resistance to a wide array of Special Attacks (Psychic in particular) it can frequently switch into opposing Special Attackers. The main thing to watch out for when using Metagross is Earthquake and Fire Attacks. While it can take a EQ chances are after taking one it won't be of much defensive use anymore, with no means of self-recovery (barring Rest) after taking an Earthquake it's tanking abilities are severely cut. This is especially important when it comes to switching into Choice Banders is a CB Earthquake has the potential to wipe it out. Fortunately for Metagross, Fire types are few in OU, but a STAB Flamethrower/Fire Blast has the potential to 1KO Metagross and an un-STABed Fire Attack will usually 2KO it, this especially important to watch out for when switching into a Pokémon like Alakazam, who has the potential to surprise you with a Fire Punch.

Leftovers allows Metagross to take more hits but gives away that it isn't the Choice Bander (the other main Metagross set). With Lum Berry you can tease that it's the Choice Bander (this is especially helpful for the element of surprise) as well as giving you added defence against surprise statuses, nothing will annoy you more than Thunderbolt's 10% chance of paralysis kicking in mid-way through sweeping. Most people go with Leftovers since Metagross is great for filling as a secondary Physical Wall but Lum Berry is an option.

EV Corner:

EVs: 140 HP / 252 Atk / 28 Def / 88 Spd
Adamant Nature (+Atk, -SAtk)

With 198 Speed, after Agility it'll outrun almost everything. A little more speed can be chosen though, for example 207 to outrun Suicune at it's Base Speed or 208 to outrun Marowak. Some Attack can be sacrificed in favour of Defences, it does well as a Physical Wall so Defence is important for blocking attacks.

Other Optional Sets

Metagross @ Choice Band
Trait: Clear Body
EVs: 132 HP / 252 Atk / 124 Spd
Adamant Nature (+Atk, -SAtk)
- Meteor Mash
- Explosion
- Earthquake
- Sludge Bomb / Rock Slide

CB Meta is the other popular set for Metagross. With it's monster attack stat, strong moves and the ability to get into play rather easily it's a very user-friendly set-up. The last move is up to the player and is usually a situational filler: Sludge Bomb has a Poison chance which is handy against bulky waters (except for Milotic) and Ludicolo, Rock Slide is super effective against Gyarados and Zapdos which both cause Metagross problems.

Metagross @ Leftovers
Trait: Clear Body
EVs: 204 HP / 52 Def / 252 SAtk
Rash Nature (+SAtk, -SDef) / Quiet Nature (+SAtk, -Spd)
- Meteor Mash
- Psychic
- Ice Punch
- Thunderpunch

Special-Sweeper Metagross. It's good as a surprise set and with Psychic STAB and a "sort of" Bolt-Beam with Ice and Thunderpunch it has a fair bit of versatility. Meteor Mash will keep Snorlax, Regice and Blissey from raining on your party.

Other Optional Moves

Rest, Sleep Talk, Reflect, Light Screen, Iron Defence, Thunderpunch, Toxic, Psych Up, Pursuit, Hidden Power [Fire]. Rest and Sleep Talk would work nicely if Metagross is going to be your main Physical Wall, it also gives you a method of removing statuses and gives your team a Sleep absorber. Reflect, Light Screen and Iron Defence are just generally good moves, better saved on a utility Pokémon. Thunderpunch is mentioned again for the physical sets, with some EV support it can 2KO Skarmory and can KO Gyarados. While everything can get Toxic it can be especially useful for use against bulky waters. Psych Up can copy opponents stats which has it's obvious uses. Pursuit is very useful in the Uber Metagame since it can finish off Shedinja and Deoxys. HP Fire is good for taking out Skarmory and Forretress.

Strategy Against Metagross

Skarmory, Forretress and bulky waters (Swampert, Suicune, Milotic and Vaporeon in particular) are the main ones. Gyarados, Zapdos and the Fire/Flying types (Charizard, Moltres and Ho-oh in Ubers) can counter it when Rock Slide is absent. Ludicolo resists both Meteor Mash and Earthquake but if Metagross has Sludge Bomb it'll probably be KOed. If Weezing can switch into Earthquake or something it resists it can scare it off with the threat of Will-o-Wisp.

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/Emerald Evolve Metang Colosseum & XD Snatched From Cipher Nascour in Realgam Tower Fire Red/Leaf Green Trade from R/S/E/C

Animé Appearences

Metagross has had several Animé Appearences. First it was in LaRousse City under the control of Hitomi. It helped defend the city from Deoxys but it did not do much else. After that, in the Hoenn League, Ash's Rival Tyson used one in many battles including his battle against Ash. After that, Frontier Brain Anabel used one in her two battles against Ash Movie 7: Destiny Deoxys! Episode 402: Like A Meowth To Flame! Episode 407: Choose It Or Lose It! Episode 408: At the End of the Fray! Episode 446: Salon Maiden Anabel Appears! Episode 447: Battle Tower! Telapathy Battle!