I will provide my best team for Pokemon X

Greninja (Water / Dark)
Ability: Torrent
- Surf
- Extrasensory
- Water Shuriken
- Night Slash / Dark Pulse
Greninja is the favorite stater of Kalos. When it comes to speed he beats most of the Kalos Pokemons and Water Shuriken helps to break Sturdy ability. Surf is unavoidable to travel in the game. Extrasensory covers fighting type Pokemons considering the speed of this guy, he can take them out. I specifically used it against Diantha's Hawlucha. Greninja learns Night Slash at level 70. Dark Pulse can be obtained from Route 16 after defeating all 8 gym leaders.
Weakness: Electric, Grass, Bug, Fighting, Fairy
Resistance: Water, Fire, Steel, Ice, Dark, Ghost
Immune to: Psychic

Venusaur (Grass / Poison)
Ability: Overgrow
- Sludge Bomb
- Solar beam
- Synthesis / Earthquake
- Petal Dance
For Greninja as starter your options for Kanto starters would be Charizard or Venusaur. My choice is Venusaur. He is super strong, good defense and he can solo fairy types with ease when mega evolved. Sludge bomb does really well when going again Mega Gardevoir. Earthquake covers steel/fairy Pokemons but you can add a fire type Pokemon in your team, and use synthesis to gain HP. Solar Beam is a good move but grass types carry only in few parts of game.
Weakness: Fire, Flying, Psychic, Ice
Resistance: Grass, Fairy, Electric, Water, Fighting

Talonflame (Fire / Flying)
Ability: Flame Body
- Flamethrower
- Fly
- Brave Bird / Steel Wing
- Acrobatics
Talonflame would not be the best Pokemon when it comes to fire type, but he really serves as best team if you want a fire type as coverage. Fly is unavoidable when you are travelling in game. Brave Bird to knock off or damage enemies bad, With it's incredible speed he can either take down his opponent or deal damage. He can solo an entire grass type gym.
Weakness: Water, Rock, Electric
Resistance: Fire, Grass, Fighting, Steel, Fairy, Bug

Lucario (Fighting / Steel)
Ability: Stead Fast / Inner Focus
- Aura Sphere
- Close Combat
- Earthquake / Shadow Claw
- Poison Jab / Dragon Pulse
For me he is best in Kalos, With many variety of move types he can solo most of the gym. He can solo Mega Gardevoir with Poison Jab in his Mega form. Earthquake for Steel/Rock coverage Or Shadow Claw for Psychic / Fairy type coverage. He doesn't learn steel type moves but definitely worth a try. You can catch Riolu early in the game and evolve or Get lucario after you defeat Shalour city gym leader.
Weakness: Fire, Ground, Fighting
Resistance: Normal, Grass, Ice, Bug, Rock, Dragon, Dark, Steel
Immune to: Poison

Garchomp (Dragon / Ground)
Ability: Sand Veil
- Dragon Claw
- Dragon Rush
- Earthquake
- Strength / Crunch
A formidable dragon type with Base of 600. This guy is the best dragon type after Salamance in Kalos Region. I did solo Elite four Drasna with this guy. Dragon Claw is a STAB, Earthquake covers Steel, Rock, Electric. Strength if you travel in Victory road and Crunch if you do not want to have a Psychic coverage separately. With his great attack and decent speed, He can solo 3 of Elite Fours in Kalos.
Weakness: Dragon, Fairy, Ice
Resistance: Fire, Rock, Poison
Immune to: Electric

Aurorus (Rock / Ice)
Ability: Refrigerate
- Ice Beam
- Thunderbolt
- Psychic
- Stone Edge
A Solid Tanker with Great HP, this Pokemon is underrated. I have used this guy again Hawlucha who has 4x coverage, and took him down with Ice Beam. He has a variety of moves and serves as a great member for defense. Obviously not a required member but he could make a solid defense when other Pokemons need to be recovered.
Weakness: Water, Grass, Fighting, Ground, Rock, Steel
Resistance: Normal, Flying, Ice, Poison
All the Pokemons I have mentioned above has unique types and never intersect. I usually pick teams of this pattern and Kalos has lots of Pokemon, and you can create your own team without duplicating types. And always have a Pokemon who can learn variety of moves which helps in Kalos. Thanks to everyone who chooses my team.