If you're running specs, then Draco Meteor. It is an extremely powerful move for le dragon types. Here is the Latios set that some people run:
Latios @ Choice Specs
Trait: Levitate
Timid Nature
EVs: 252 SpA, 252 Spe, 4 SpD
Moves:
Draco Meteor
Psyshock
Hidden Power Fire
Surf
Here are some calcs proving how strong DM Latios is:
>252 SpA Choice Specs Latios Draco Meteor vs. 0 HP / 4 SpD Mega Charizard Y: 282-333 (94.9 - 112.1%) -- 68.8% chance to OHKO
>252 SpA Choice Specs Latios Draco Meteor vs. 252 HP / 0 SpD Mega Venusaur: 271-321 (74.4 - 88.1%) -- guaranteed 2HKO
Hope I helped!