Dialga is an extremely useful Pokémon in the Uber environment because of its wonderful Dragon/Steel typing that gives him many useful resists, especially against the very common water type moves and only 2 weaknesses. Furthermore, it has a massive movepool and can act as an all-out special sweeper, as a Trick Room inducer or even as a physical set up sweeper. This versatility is what makes Dialga one of the most threatening Pokémon in VGC 2010 along with his brilliant base stats.

Base Stats:

HP: 100
Att: 120
Def: 120
Sp Att: 150
Sp Def: 100
Spd: 90

Moveset Explaination

Dialga's mind-blowing versatility as a sweeper is reflected by the given moveset. Always make sure to adjust your moves to your item choice.

Dragon Pulse is Dialga's primary attack move because it deals solid damage to nearly all Pokémon in the metagame including OHKOs on Rayquaza, Latios, Kingdra, Palkia (with Choice Specs) and 2HKOs on Palkia (without Choice Specs), Kyogre (Specs), Cresselia (bold 252/0) and Giratina.

Aura Sphere gives you a handy tool to fight other Dialga and Tyranitar. Be aware that the former can carry a Chople Berry, however, in which you are better off just firing Dragon Pulses. Timid Choice Specs Aura Sphere can 2HKO 252/0 Metagross.

The third move depends on your item choice. If you use Choice Specs, Draco Meteor is an excellent choice because it OHKOes the Dragons mentioned above who live through a Dragon Pulse. The already impressive power of Specs Draco Meteor is further boosted by Helping Hand support which makes it so that Dialga has a shot at OHKOing even specially defensive Snorlax! They need to max out HP and at least 124 SpDef to survive for sure. If you opt to not use a Choice item, Protect is the best option just because it is one of the most useful moves in Double battling and in addition to that the opponent wastes 2 PP thanks to Pressure if he tries to attack a protecting Dialga.

The fourth move offers options to satisfy the user's preferences. Thunder is useful for killing water Pokémon and for its chance to paralyse while the fire moves deal with specially defensive Abomasnow that are not killed by a non-Specs Aura Sphere as well as with the omnipresent Shedinja. A notable fact about Overheat is that it will kill most Groudon in the sun when Dialga is equipped with Choice Specs but some may dislike the SpAtk drop in which case you can stay with Flamethrower. Toxic works well with Protect and is an option for a more defensive approach together with Leftovers as item. Magnet Rise is a surprise option and can remove all of Dialga's weaknesses temporarily when you equip it with a Chople Berry. However, the free turn for the opponent and the fact that the effects disappears upon switching may put some players off, but it is a smart move if the opponent only has Ground moves to threaten Dialga.

Notes

For the sweeping Dialga, 252 EVs in Speed and Special Attack and a Timid nature are recommended because then it at worst ties with non-Scarf-Kyogre and Groudon which is noteworthy because you can lower their speed with your partner's Icy Wind. It is faster than Modest Palkia and Adamant Rayquaza as well and outspeeds Adamant Garchomp too. Do not worry about power because with Choice Specs it still deals massive damage even if Timid. For a more sturdier and defensive approach on a non-Specs set you can change the nature to Modest and the EV spread to 220 HP / 4 Def / 36 Spe / 92 SpAtk / 156 SpDef. You still have a good punch while you are decently bulky on the specially defensive side. Some speed is used to outspeed other bulky Pokémon.