Welp, everyone here already named mines.
Sceptile, Cloyster.
Pinsir with Quick Attack ain't half bad either.
To name some new ones..
Mega Pidgeot: Flying, Fighting Ability: Intimidate
Mega Nidoking: Ground, Poison Ability: Sheer Force
Mega Arcanine: Fire, Fighting Ability: Flare(Pokemon that make contact are 100% burned)
Mega Kingler: Water, Steel Ability: Crusher(Moves that make physical contact have a 50% chance to Flinch, Confuse, or Paralyze)
Mega Crobat: Flying, Poison Ability: Venom Dipped(All attacks induce weak poison)
Mega Steelix: Steel Ability: Stance Change
Mega Sceptile: Grass, Dragon Ability: Faunalate(the stage condition "grassy terrain" is summoned)
Mega Shedinja: Ghost, Bug, Fairy Ability: Cursed(Will live for 3 turns but guaranteed death afterwards)
Mega Sharpedo: Dark, Water Ability: Rough edge(Physical attacks will have the same effect as rough skin and rough skin is still in effect)
Mega Flygon: Ground, Dragon Ability: Sand Force
Mega Salamence: Dragon, Electric Ability: Levitate
Mega Luxray: Electric, Dark Ability: Speed Boost
Mega Honchkrow: Dark, Flying Ability: Bossy(Murkrows assist in attacks giving random status conditions)
Mega Spiritomb: Dark, Ghost Ability: Foresee(Reveals and Pokemons moves every turn)
Mega Toxicroak: Fighting, Poison Ability: Battle Stance(Increases accuracy on all moves and increases Defense and S.Defense by 25%)
Mega Weavile: Ice, Dark Ability Shadows(This Pokemon has the immunities resistances and weakness of Ghost types)
Mega Yanmega: Bug, Dragon Abilty: Speed Boost
Mega Scolipede: Poison, Steel Ability: Arrogant(Any move that lowers stats with increase all stats by 1 stage)
Mega Krookodile: Dark, Ground Ability: Thieving(Steals the item of the Pokemon it first encounters permanently)
Mega Crustle: Rock, Bug Ability: Chipped Rock(Attacking increases speed and attack)
Mega Scrafty: Dark, Fighting Ability: Fear Instill(lowers enemies attack and defense but inceases speed)
Mega Chandelure: Ghost, Fire Ability: Inhabit(Takes over different household furniture during battle, changing its typing)
Mega Mandibuzz: Dark, Flying Ability: Lazy(Heals 20% of HP every turn)