Dialga, the ruler of time, has had a tumultuous history. One of the best Pokémon introduced in its debut Generation, Dialga's main claim to fame is its Dragon Steel typing. A Dragon type not weak to Ice and Dragon was rare and Dialga had the defenses to prove immensely frustrating to take down. This boon kept Dialga in relevance when Fairies were introduced, as even Xerneas does not like eating STAB Flash Cannons from Dialga. However, as the generations passed on, more and more legendary Steel types like Solgaleo, Dusk Mane Necrozma, and Zacian Crowned were added, giving Dialga competition that it did not really have before. And while Dialga isn't weak to Fire like other Steel types, it doesn't resist it either. And unlike other Steel types, Dialga doesn't resist Fairy or Dragon which is worrying considering the immense power creep over the generations, that has seen its bulk turn less and less impressive.
Dialga remains eternal. Its defensive profile is unique and has a wide and varied Special movepool that gives Dialga an option for just about anything. Its bulk has it act as an excellent Stealth Rock setter and has Trick Room as an excellent Speed Control tool.
Two Hands to a Clock
Dialga
Ability: Telepathy
Item: Adamant Orb
-Roar of Time
-Flash Cannon
-Earth Power
-Trick Room
Nature: Modest (+SATK, -ATK)
EVs: 252 HP / 252 SATK / 4 SDEF
Thanks to its immense bulk and typing, Dialga remains a persistent threat on the field in a doubles format. Roar of Time and Flash Cannon comprise Dialga STAB moves. Roar of Time to be used while Dynamaxed or to reach a KO, while Flash Cannon is more reliable and both are complimented by the Adamant Orb. Earth Power hits Steel and Fire types hard, and compliments Dialga's STABs immensely while it is Dynamaxed, enabling Dialga to snowball defensively.
Trick Room may be odd on Dialga, whose base 90 speed is high for a Trick Room user, but it functions as an excellent form of Speed control. Against universally faster teams, Dialga can pop a Trick Room to swing momentum into its favor, while having enough Speed that an ally with Tailwind will enable an uninvested Dialga (assuming perfect Speed IV) to outspeed a Timid Dragapult, and thus most of the unboosted metagame.
Telepathy enables Diagla to play well with Pokémon that have damaging spread moves like Earthquake, Lava Plume and Discharge.
Other Options
-Blizzard -hits Groudon and Landorus-Therian hard and provides weather control while Dynamaxed.
-Fire Blast -melts Zacian and Amoongus
-Thunder -punishes Kyogre and Rain