Okay. Here we go. No matter what type of Salamence you choose to use, it needs a Speed Boosting nature to survive in competitive play. It's best stat is its Attack, but its Special Attack is respectable. So, in order to be able to use both stats, and to gain Speed, it needs to use a nature that boosts its speed.
So, the best options for Salamence are:
Pure Physical Salamence (Jolly- Speed +, Special Attack -)
Pure Special Salamence (Timid- Speed +, Attack -)
Mixed Salamence (Hasty- Speed +, Defense -)
(Naive- Speed +, Special Defense -)
I hope that clarified things for you.