I'm trying to fine-tune it before the VGC.
I have some experience with last year's VGC rules, but not so much with the rules of 2010, so while I tried to keep new threats like Kyogre and Groudon in mind, it is quite likely that I am unaware of certain things.
All suggestions or criticisms are welcome!
Nature: Sassy (+Sp. Def, -Spe)
Trait: Own Tempo
EVs: 252 HP // 252 Sp. Def // 6 Def
~Follow Me
~Fake Out
~Dark Void
~Super Fang
Item: Focus Sash improves the likelihood that Smeargle will survive the first round so that after supporting Dialga's setup, it can use its other moves.
Nature and EVs: During the first turn, Smeargle will be using priority moves (Fake Out, Follow Me) that don't require any investment in Speed. Once Trick Room is active, Smeargle's low Speed will allow it to strike first. 252 EVs were placed in Special Defense to help take the more common type of attacks better. The leftover EVs were placed in Defense.
Moveset: Follow Me absorbs Taunts and Fake Outs aimed at shutting down Dialga's attempt to set up a Turn 1 Trick Room. Fake Out is good for slowing down opposing leads that are unlikely to be carrying disruptive moves. (For example, against a Garchomp/Zapdos lead, Fake Out would be used on Garchomp.) Dark Void can put both opponents to sleep, making it easier to attack without fear of retribution. Super Fang halves an enemy's health so that Dialga has an easier time finishing it off.
Nature: Quiet (+Sp.Atk, -Spe)
Trait: Pressure
EVs: 252 HP // 252 Sp. Atk // 6 Def
~Dragon Pulse
~Thunder
~Trick Room
~Protect
Item: Lum Berry assures that Dialga's Trick Room won't be interrupted by Freeze, Paralyze, or Sleep.
Nature and EVs: Maxing Special Attack is important to deal as much damage as possible, and the HP EVs give it some degree of durability to keep it in the game long enough to deal damage and possibly even set up a second round of Trick Room. The remaining EVs are in Defense because Earthquake is a bigger threat to this Pokemon than things like Thunder and Dragon Pulse.
Moveset: Dragon Pulse is a solid STAB attack that will hit super-effectively against many of the Uber Pokemon that are allowed this year. Thunder is a secondary attack that works quite nicely in the rain. Trick Room is obvious. It allows all of my slow, hard hitting Pokemon to go first for 4 turns. Protect is a great move for scouting, and is important for those rare occasions when I have both Metagross and Dialga out at the same time. (Metagross has two moves that affect all Pokemon on the field, making it important for Dialga to be able to protect itself from its ally.)
Nature: Quiet (+Sp. Atk, -Spe)
Trait: Levitate
EVs: 252 HP // 252 Sp. Atk // 6 Sp. Def
~Shadow Ball
~Dragon Pulse
~Will-o-Wisp
~Protect
Nature and EVs: Like Dialga, I focused on boosting its offensive and defensive stats, because Speed isn't needed. The remaining EVs were placed in Special Defense to help it survive Dragon Pulses and such.
Moveset: Shadow Ball and Dragon Pulse are good STAB attacks that hit super-effectively against Dragons and Psychic types, which will be quite common. Will-o-Wisp kills the rare Shedinja and allows me to cripple physical attackers like Tyranitar and Metagross. Protect is useful for scouting, and helps Giratina-O stay in the game longer.
Nature: Brave (+Atk, -Spe)
Trait: Clear Body
EVs: 252 HP // 252 Atk // 6 Sp. Def
~Meteor Mash
~Earthquake
~Thunderpunch
~Explosion
Item: Shuca Berry helps Metagross survive Ground-type attacks that hit super-effectively. It was chosen over Occa Berry because Rain will probably be a common weather effect, so Fire-type attacks will already be weaker.
Nature and EVs: Again, I boosted Metagross's offensive and defensive abilities because Speed isn't needed during Trick Room. The remaining 6 EVs were placed in Special Defense because its held item is already lowering damage from Earthquakes, the only Physical move it really fears.
Moveset: Meteor Mash is a standard high powered STAB attack. Earthquake is there for coverage, and because it has a nice synergy with its partner, Giratina-O. Thunder Punch is mainly for Kyogre. Explosion is a powerful finishing blow that can't damage Giratina-O (and if it's in battle with Dialga, Dialga can use Protect.)
Testing: Max speed Smeargle
Thanks for reading!