Teams for all starters. All are decent.
Greninja team: [Greninja / Charizard / Gardevoir / Lucario / Scolipede / Heliolisk]
Delphox team: [Delphox / Gyarados / Staraptor / Lucario / Scolipede / Heliolisk]
Chesnaught team: [Chesnaught / Gyarados / Gardevoir / Pyroar / Scolipede / Staraptor]
HM Slave: Bibarel / Corphish [Cut / Strength / Surf / Waterfall]
Since EV training is easy in XY, I would give you EVs, but it isn't necessory
Greninja team:
Greninja
Ability: Torrent
EVs: 128 Atk / 252 SpA / 132 Spe
- Surf / Waterfall [HM]
- Dark Pulse [TM - Route 16] / Night Slash [lvl 70 / Heart Scale]
- Ice Beam [TM - Defeat Wulfric]
- Extrasensory [lvl 49] / Acrobatics (if no held item) [TM - Coumarine City] / Water Shuriken [lvl 36]
Any nature except those which depletes attacking stats and Speed.
Location: Starter
This thing is really a beast with a very versatile movepool. First option is definitely a slot for HM which Greninja can use effectively in battles, second option is for dark stab. Ice Beam is for coverage, and is extremely useful for... Dragons. Extrasonsory and Acrobatics are Fighting coverage. Water Shuriken breaks through sturdy (to hit some annoying gravelers).
He can pass through Viola with no difficulty. He is capable of sweeping Grant, Olympia, Malva and Drasna. Due to high speed he is all time fan favorite.
X
Charizard @ Charizardite-X
Ability: Tough Claws
EVs: 252 Atk / 252 Spe
- Flare Blitz [lvl 77 / Heart Scale]
- Earthquake [TM - Route 22]
- Dragon Claw [TM - Victory Road]
- Fly [HM]
Any nature which doesn't deplete Atk and Spe.
Location: Lumiose City
Tough Claws is actually great. Fire stab is obvious, Flying stab is actually good even for battles when not mega-evolved. Dragon Stab is obvious. Earthquake is coverage.
Y
Charizard @ Charizardite-Y
Ability: Drought
EVs: 252 SpA / 252 Spe
- Flamethrower [lvl 47] / Heat Wave [lvl 71 / Heart Scale]
- Fly [HM]
- Solar Beam [TM - Route 21]
- Focus Blast [TM - Anistar City]
Any nature which doesn't deplete SpA and Spe.
Location: Lumoise City
With 150 SpA and Drought, this thing is really a beast after Mega Evolving. Fire Stab and Flying stab are obvious, Solar Beam abuses the sun, and can take on Siebold, believe me. Focus Blast is just for coverage (against rock types).
Main flyer of the team. Useful against Ramos, Wulfric, Drasna (X) and Siebold (Y).
Gardevoir
Ability: Synchronise / Trace (latter is preffered)
EVs: 252 SpA / 252 Spe
- Moonblast [lvl 85 / Heart Scale]
- Psychic [lvl 40]
- Calm Mind [lvl 33]
- Shadow Ball [TM - Terminus Cave] / Energy Ball [TM - Winding Woods] / Thunderbolt [TM - Defeat Clemont] / Focus Blast [TM - Anistar City]
Any nature which doesn't deplete SpA and Spe.
Location: Route 4
Gardevoir is also decent, is able to take on Korrina where you can't evolve your Charmeleon. Moonblast is must for every Fairy type. Psychic is other stab. Calm Mind to boost your stats, it is replaceable by the rest of coverage options. Catching a Ralts is relatively easier than getting a Pixilate Sylveon, though you may encounter a headache while you search for Ralts. Worth the effort.
Lucario @ Lucarionite
Ability: Adaptability
EVs: 128 Atk / 252 Spe / 132 Spe
- Aura Sphere [lvl 42] / Close Combat [lvl 55]
- Psychic [TM - Pokemon Village]
- Bone Rush [lvl 29] / Earthquake [TM - Route 22]
- Dark Pulse [TM - Route 16] / Dragon Pulse [lvl 60] / Stone Edge [TM - Frost Cavern]
Any nature which doesn't deplete Atk, SpA and Spe.
Location: Tower of Mastery
Save before battling Serena so that you can reset if you get a bad natured one. Who doesn't need 145 / 140 attacking stats with decent speed? Lucario is the real hero here, thanks to Korrina who has the big heart to give you her Lucario with Lucarionite and Mega Ring. Aura Sphere and Close Combat hits like hell with Adaptability Stab. The rest are for coverage.
Scolipede
Ability: Poison Point / Swarm
EVs: 252 Atk / 252 Spe
- Megahorn [lvl 65 / Heart Scale]
- Poison Jab [TM - Shalour City]
- Earthquake [TM - Route 22]
- Rock Slide [TM - Couriway Town] / Stone Edge [TM - Frost Cavern]
Any nature which doesn't deplete Atk and Spe.
Location: Route 6
Thanks to introduction of Fairy types, this beast got some importance in the game. Megahorn hits very hard, Poison Jab is Stab, the rest are coverage.
Heliolisk
Ability: Dry Skin / Sand Veil (former is preferred)
EVs: 252 SpA / 252 Spe
- Thunderbolt [TM - Defeat Clemont]
- Surf [HM]
- Grass Knot [TM - Coumarine City]
- Dark Pulse [TM - Route 16] / Flash [TM - Reflection Cave]
Any nature which doesn't deplete SpA and Spe.
Location: Route 9
Heliolisk is an awesome mon which can easily take care of the abundant water types. It gets access of Surf, which is actually cool. Thunderbolt is stab. Grass Knot to take care of Water / Ground types such as Quagsire. Dark Pulse is just coverage. Flash is for illuminating caves.
Delphox team:
Delphox
Ability: Blaze
EVs: 252 SpA / 252 Spe
- Flamethrower [lvl 42]
- Psychic [lvl 51]
- Grass Knot [TM - Coumarine City] / Solar Beam [TM - Route 21]
- Sunny Day [lvl 55]/ Cut [HM]
Any nature which doesn't deplete SpA and Spe. Despite having poor movepool, he takes care of Viola easier than the rest, and can handle Korrina. He also has higher SpA stat. Flamethrower and Psychic are stab, Grass Knot and Solar Beam are coverage and Sunny day to support Flamethrower and Solar Beam. Cut is an option if you want to cut through those trees.
Gyarados
Ability: Intimidate
EVs: 252 Atk / 252 Spe
- Dragon Dance [lvl 44]
- Waterfall [HM] / Aqua Tail [lvl 35]
- Ice Fang [lvl 32] / Stone Edge [TM - Frost Cavern]
- Earthquake [TM - Route 22] / anything
Any nature which doesn't deplete Atk or Spe.
Location: Route 3 (Old rod)
It is obtained as soon as you get an old rod. You can't miss Intimidate, right? Gyarados is a beast since Gen 4, and thank god, its already high Atk stat and access to Dragon Dance makes it a beast. Waterfall / Aqua Tail is stab. Ice Fang is for Grass Coverage, and most of the mons have 4x weakness to Ice. If you want more power, you can go with Stone Edge or Rock Slide. Earthquake is electric coverage.
Even though you need to return to Route 3 as soon as you receive an Old Rod, even though you need to grind it like hell to evolve, it is extremely useful for this team.
Staraptor
Ability: Intimidate / Reckless (former supports your team, latter makes sure it hits harder)
EVs: 252 Atk / 252 Spe
- Brave Bird [lvl 49]
- Close Combat [lvl 34]
- Fly [HM]
- Return [TM - Lumiose City]
Any nature which doesn't deplete Atk and Spe.
Location: Route 11
My favorite bird which gets access to Close Combat, and having 120 Atk. Talonflame surely is available earlier, but the type overlap.... Coming to the moveset details, Brave Bird is primary stab, Fly is for HM purpose, Return for neutral stab. Close Combat hits Rock, Ice and Steel types (except Aegislash). I recommend Intimidate Staraptor, since obtaining reckless is a little difficult (rare + low level).
Hits like a truck. Your flyer.
All three are same as above.
Chesnaught team:
Chesnaught
Ability: Overgrow
EVs: 252 Atk / 252 Spe or 252 Atk / 252 HP
- Brick Break [TM - Terminus Cave] / Hammer Arm [lvl 60 / Heart Scale]
- Seed Bomb [lvl 35] / Wood Hammer [lvl 55]
- Bulk Up [lvl 44]
- Poison Jab [TM - Shalour City] / Dig [TM - Shalour City]
Any nature which depletes SpA. He is a nice Physical tank. First two moveslots for fighting stab and a grass stab respectively. Bulk Up to make it more tanky. The rest is for coverage. Dig can also be used to escape dungeons.
Pyroar
Ability: Rivalry / Unnerve (former not recommended)
EVs: 252 SpA / 252 Spe
- Flamethrower [lvl 38]
- Hyper Voice [lvl 48]
- Dark Pulse [TM - Route 16]
- Hidden Power [TM - Anistar City] / anything
Any nature which doesn't deplete SpA and Spe.
Location: Route 22
Flamethrower and Hyper Voice are stab, Dark Pulse is coverage and Hidden Power is for additional coverage, replace it for anything in case you got a bad type.
Viola would be extremely difficult if you only have your starter at that point. So I recommend to catch a Lilteo for that matter.
The rest are same as above.
Hope this helps :)