Palkia
First of all, let’s compare the two. Dialga is a Dragon/Steel type meaning when he has a Steel type move or Dragon type move it is powered up

However, they also have their differences. Palkia has more Speed then Dialga (due to his Steel or Palkia’s Water type I’m sure), but Dialga’s Roar of Time has more power then Palkia’s Spacial Rend. When it comes to learning moves they have Metal Claw and Water Pulse at level 10, Roar of Time and Spacial Rend at level 40, and Flash Cannon and Aqua Tail at level 80. Weakness is little when it comes to Palkia as his only weakness is Dragon while Dialga is weak against Ground, Fighting, and Dragon types. When it comes to resistance Dialga is uneffected by Poison while Grass, Normal, Water, and Electric deal half their damage. Palkia has no immunity to any type but Fire, Water, and Steel type moves deal half their damage.
There are many items they could hold in order to power up or weaken a Super Effective move. Berries, Bands, and their orbs all could boost attack, defense, etc. Or just Dragon, Water, Steel, etc. There are way too many to name, but I would suggest having Palkia or Dialga hold their orb which boosts the power of their Dragon and Steel/Water moves.
Natures and Personalities of Pokémon affect what grows most and by how much. Hardy doesn’t affect any stat while Lonely raises attack most when leveling/when its favorite Poffin is given to it and slowly raises defense when leveling/Foul Poffin is given to that Pokémon. Personalities are kinda the same. Loves to Eat raises Heath Points (HP) while Highly Curious will raise its Special Attack. As there are 25 different Natures and 30 different Personalities, I won’t name all of the combos.
So which is stronger? The movie Rise of Darkrai