Mega Salamence and Mega Metagross are almost equally the same. M-Metagross has higher Defenses, but M-Salamence has higher Speed. I would recommend M-Salamence, because of his sweeping capabilities.
EVs: 252 Atk, 252 Spd, 4 Def
-Earthquake/Brick Break/Steel Wing/Roost/Rock Slide
The reason Intimidate is better for Mega Mence is becuase it lowers their attack before Mega Evolving, whereas Moxie will do nothing before Mega Evolving. Return is changed to a Flying-type move and will deal tons of damage, if Salamence likes you. If he doesn't, then Double-Edge is a better option. There are tons of options for the third move, Earthquake is incredibly strong, Brick Break is for Ice and Rock, Steel Wing is for ice and Fairys, Roost is to heal after Double Edge, and Rock Slide is again, for Ice. You can choose between these because they are all TMs. But, what I would recommend is Earthquake or Brick Break because Brick Break deals with Ice and Rock well, and Earthquake is a powerful move. The reason I didn't recommend Steel Wing (as it deals with fairys too) is becuase it's not that good of a move. Even with the Defense boost, it still only has 90 accuracy and 70 power.
And, of course, Dragon Dance is to setup and sweep.
Hope I helped!