Pokémon X and Y Walkthrough, Pokémon Move Sets: Aggron
(Please note that the recommendations below are largely made for in-game play. An Aggron in a competitive environment will likely make use of different moves to accommodate for smarter, more adaptive opponents. Want to take your Aggron a different direction? Toss your moveset in the comments.)
One of the standout bruisers from the third generation, Aggron is as much of a beast as it looks. There aren't many pokémon which can match its physical fortitude, and its raw muscles aren't anything to sniff at either. It can be brought down, though, and this article will help you craft an Aggron that can cover its weaknesses.
Type: Rock / Steel, Steel (Mega Aggron)
Aggron begind life as the tiny Aron. Raise Aron to level 32 to get a Lairon, and keep levelling to 42 to get an Aggron. You'll need Aggronite for Mega Aggron fun.
Bug, Dragon, Fairy, Flying, Ice, Normal, Psychic, Rock. Immune to Poison.
Mega Aggron: Bug, Dragon, Fairy, Flying, Grass, Ice, Normal, Psychic, Rock. Immune to Poison.
Fighting (4x), Ground (4x), Water.
Mega Aggron: Fighting, Fire, Ground.
Aggron has some rather predictable stats for a steel- and rock-type, leaning very much towards the physical. Its crowning stat is Defense, which is so massive that you probably don't even need to bother EV training it. Attack comes in second at a hefty 110 base. HP is next, and though it's not massive, it's respectable enough. Special Attack and Special Defense fall behind the pack at below average, and Speed comes in dead last. Special Defense will be the biggest problem here, as there are some really common moves which target Special Defense and can immediately wipe out your poor Aggron.
As for Mega Aggron, well, yeah. Its Attack and Defense stats skyrocket to ridiculous heights, while the rest don't change a whole lot. It would be nice if Special Defense went up more, but a little bump is better than nothing.
Sturdy: If the pokémon is at full HP and is hit by a move that will knock it out, it will instead be reduced to a single HP. If you don't plan on training Aggron's Special Defense, Sturdy is the way to go. What an annoying, annoying ability.
Rock Head: Moves that normally cause recoil damage to the user are now harmless. I personally prefer this, as it allows Aggron to use Double Edge without fear. One of the few cases where your staple normal-type move isn't Return.
Heavy Metal: The user's weight is doubled. In most cases this is beneficial as it allows moves that calculate damage by weight to hit even harder... but if your Aggron gets hit by Grass Knot, it's going to take a loooooot of damage. This is a hidden ability.
Filter: Super effective moves used on the pokémon do less damage than normal. Your Aggron is now even tougher to kill. This ability is restricted to Mega Aggron.
Aggron is an offensive powerhouse that can take a major lickin', and its movepool is best utilized with a variety of attacking moves. Your primary weapons for STAB damage will probably be Heavy Slam (particularly if your Aggron has Heavy Metal) and Stone Edge; Smack Down is a possible alternative for Stone Edge if you prefer a reliable accuracy, though it comes with a large drop in attack power. Only do this if you max out Attack EVs. (I like Rock Slide, personally, which is a bit of an intermediary between the two and can wipe out flying-types that use Fly.) Beyond this you can load down your Aggron with other move types to improve its range of use. Double Edge is amazing if you have Rock head, Brick Break is nice for wiping out barriers, Earthquake is always amazing, and Dragon Claw is a surprise dragon-type killer.
Alternatively, you can sacrifice some of the above moves for others that are more debilitating tham outright destructive, and I prefer this route. Thunder Wave is the top contender for irritating enemies. Roar is nice, Taunt can be great if you're facing a foe that relies on physical moves, and Swagger comes with less risk if confusion doesn't stick as well as you'd like. Just make sure you keep at least one attacking move - otherwise you're just wasting Aggron's offensive capabilities.
One thing to bear in mind: when Aggron changes to Mega Aggron, it becomes a pure steel-type. Consequently, it will lose STAB damage on its rock-type moves. The jump in Attack usually makes up for this loss, but not always.
EV / Super Training
My preference for EV training an Aggron is to focus on Attack and Special Defense. Defense is already massive enough to take care of itself, and you can really surprise an opponent that relies on special attacks by not falling to, say, Surf in a single hit. Special Defense has less priority if your Aggron has Sturdy as an ability. HP is a third possibility, again assuming that your Aggron doesn't have Sturdy. The other stats (Speed and Special Attack, primarily) shouldn't concern your EV training at all.
Catching an Aggron
You can't find an Aggron in the wild, but you can snag Arons and Lairons in Pokémon X. Aron appears in hordes in Terminus Cave, while Lairon appears on Route 18 and Terminus Cave on its own. Catch either and get the resulting pokémon to level 42 for an Aggron.