It's a quiet nature. + Special Attack, - Speed. It's pretty much Dialga's best nature. While he won't attack first very often, he wasn't going to do that anyways since his base speed is only 90.
It allows you to go with a bulky support/defensive role (base stats of 100 HP, 120 Def, 100 special defense, and steel/dragon typing allow that with any neutral defensive nature) while still having the power to take out offensive threats thanks to its insane special attack stat. Or to play the role of of a pure mixed attacker which, again, has bulk. (Base offense stats for dialga: 120 attack, 150 special attack.)
Again, the IVs, disregarding speed.
HP: 31
Attack: 29-30
Defense: 26-28
Special Attack: 28-29
Special Defense: 28-29
Base stats for Dialga are:
HP: 100
Attack: 120
Defense: 120
Special Attack: 150
Special Defense: 100
Speed: 90
Type: Steel/Dragon.
Nature is +Sp Att, - Spe
Those IVs are pretty decent for either a support set or a bulkier varient of the mixed attacker, but just a warning: the loss in Speed is going to HURT when it comes to speed ties vs stuff like Impish Groudon. As someone who plays a lot of Ubers, there is one CRAZY solution... using this Dialga on an Ubers Trick Room team. Dialga is pretty much the STAR of Ubers Trickroom. Dialga is reallly powerful, but it has a problem with getting revenge killed... which Trick Room solves pretty nicely! Now unfortunately, your Speed IV means that you won't under speed some stuff like Skarmory and Metagross... thankfully, Metagross is fairly uncommon in Ubers atm (despite it being pretty decent), and Skarmory is a poor answer to any Dialga that isn't setting up Bulk Up. The set is Trick Room / Draco Meteor / Outrage / Fire Blast with a Life Orb. The evs are trickier. 4 HP / 252 Atk / 252 SpA is the best when it comes to sweeping power, because after you get rid of the Steels, Draco Meteor + Outrage is gonna hurt like HELL! However, if you are okay with sacraficing sweeping power, you can use 252 HP / 4 Atk / 252 SpA to ease the set up.
As for what to use with Trick Room Dialga, usually the other big Trick Roomer is Palkia. Base 100 Speed may seem way too fast for Trick Room, but when Palkia is Quiet and has a low Speed IV, it'll still underspeed most Uber threats, while giving you that all important switch into Kyogre's Choice Specs Water Spout. You also got a lot of slow base 90s that work well on Trick Room like Groudon, Kyogre, and Zekrom. Finally, theres some odder stuff that works well on Ubers Trick Room, like Solosis (the set being level 1 Focus Sash Endeavor / Trick Room / Toxic / Protect. It MAY seem like stupid gimmick, but this thing can save you from sweeps once the ghosts are dead, and Magic Guard means Stealth Rock and Sand wont get rid of your Focus Sash) and Choice Band Ferrothorn (base 94 Atk may not seem like much, but with Gyro Ball hitting a lot of Ubers at the full 150 base power and Power Whip, you can actually 2HKO a lot of Ubers!).
Of course, as much as I like Trick Room in Ubers, your Dialga is still fine for the usual support or bulky mixed sets AS LONG as you have something for scary base 90s like Groudon and Choice Specs Kyogre!