How do I choose the best nature for my Pokemon?

Question

Which stats do I increase and decrease?

Something like this

Dragalge

Hp65 attack75 defense90 sp attack97 sp defense123 speed44

Groudon

Hp100 attack150 defense140 sp attack100 sp defense 90 speed90

Aegislash blade forme

Hp60 attack150 defense50 sp attack150 sp defense 50 speed60

Answer 1

Competitively speaking its best to not try to balance out stats but to enhance those that are good. The best option is often to decrease the attacking stat you won't use and increase what you need the most. It depends also on the moveset however. Infernape can use both Adamant, Jolly, Modest and Timid since it can run both physical and special sets.
In the case of Dragalge, Calm is the best to allow it to take more special hits at the cost of its Attack which it shouldn't need or use anyway.
As with Groudon it depends a lot on the set you are running. Adamant is preferred for a offensive variant, Impish for a defensive.
For Aegislash the case is the same. Physical variants are the most common and prefer Adamant, but it can still use Modest thanks to its good Spatk. However you need to plan your moves and sets accordingly in this case.