Return + Dragon Dance!
Both are normal-types and physical attacks.
Return has a maximum of 102 BP.
Double Edge has 120 BP and recoil-damage.
M-Salamence gets Aerilate that increases the power of Normal-type moves by 30%. It then changes those moves to Flying-type.
>Double Edge = 120 BP + 30% = 156 x 1,5 (STAB) = 234 BP (351 BP with Dragon Dance) with recoil damage.
Return = 102 (Maximum happiness) BP + 30% = 132 x 1,5 (STAB) = 198 BP (297 BP with 1 Dragon Dance) without recoil.
M-Salamence has great Attack, gets from 135 base Attack to 145 and the new ability Aerilate makes all normal types to a flying type and makes M-Salamence very strong and agressive with a STAB +30%, now with Dragon Dance!
In my opinion, Return is the best, boosted with maximum happiness, because it won't get recoil damage and Double Edge would take your HP by almost half.
Also you can't hold any item like Leftovers, remember this is a Mega-Pokémon!
M-Salamence can learn others normal-types such as Headbutt, Hyper Beam, Return, Facade, Giga Impact, Strength, Hyper Voice, Body Slam. (+ Return and Double Edge)
Body Slam is also a great choice!
If you don't want to use Return because of happiness, Body Slam has a 85 BP and has a 30% chance of paralyzing the target.
If the target has used Minimize, Body Slam will never miss and its base power is doubled to 170.
Hope this helps.