By a wide margin, Salamence.
On its own its a pretty terrifying Pokemon, sporting above average stats with none being lower than 80, a diverse and functional movepool with access to such great moves as Dragon Dance, Outrage, Roost, Hydro Pump, Draco Meteor, Flamethrower, Earthquake and Stone Edge, a good typing in Dragon/Flying that resists 5 other types and a gives access to powerful STAB Dragon moves, as well as two highly useful abilities in Moxie and Intimidate.
What set Salamence apart however its Mega Evolution. It still have access to its great movepool, allowing it to stall, attack physically/specially/mixed and keeping its typing. By getting a incredible stat boost, most notably 120 Spe and 145 Attack and a new and absolutely lethal ability that turns it's already good Returns/Double Edges/Facades into monstrous STAB flying moves of doom Salamence becomes a force of nature, being able to plow through most normal teams with ease.
Salamence's great offensive stats and wide movepool allow it to run almost any set imaginable, but I will showcase a few here:
Standard DD Mega (Salamence) @ Salamencite
Ability: Intimidate
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Dragon Dance
- Return
- Substitute
- Roost
Special Mega (Salamence) @ Salamencite
Ability: Intimidate
EVs: 252 HP / 252 SpA / 4 SpD
Modest Nature
- Hyper Voice
- Roost
- Draco Meteor
- Flamethrower
LO Moxie Sweeper (Salamence) @ Life Orb
Ability: Moxie
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Dragon Claw
- Dragon Dance
- Earthquake
- Roost
Choice Band Wallbreaker (Salamence) @ Choice Band
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Outrage
- Earthquake
- Fire Fang
- Iron Tail
Special Sweeper (Salamence) @ Life Orb
Ability: Intimidate
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Draco Meteor
- Flamethrower
- Hydro Pump
- Roost
Staller/Support (Salamence) @ Leftovers
Ability: Intimidate
EVs: 252 HP / 4 Def / 252 SpD
Calm Nature
- Roost
- Toxic
- Substitute
- Defog
Source: http://pokemondb.net/pokedex/salamence