I'll do one with my personal favorite Kalos starter, Froakie
Greninja @ Mystic Water
Availability: Aquacorde Town (gift from Tierno, Starter)
The best starter for Kalos came down to Greninja because of it's excellent Speed and good Special Attack. Only downside though, are its defenses, which are poor by comparison. Water-typing is decent for route sweeping but is not always spectacular in major battles. It gains Dark-typing upon evolving to Greninja which is not useful until the Move Relearner. Hydro Pump is your Water STAB move, but keep Water Shuriken until you get Hydro Pump, whichever works for you. Dark Pulse is your Dark type STAB move, but keep Night Slash until you get the Dark Pulse TM. Coverage for Greninja would be Ice Beam for the Grass and Dragon types. And Extrasensory for the Poison and Fighting types. As for it's major battles, Greninja deals with Olympia thanks to its Psychic immunity and STAB Night Slash (Before Dark Pulse), and its good for Grant's gym. Other than that, it lacks favorable match-ups and is particularly poor against Ramos, Valerie and Clemont. It fares better against the Elite Four thanks to its excellent coverage, and it is good for Team Flare's numerous Houndours/Houndooms. Overall, Froakie is a solid choice thanks to its availability, strong STAB moves, and stat distribution. However, its lacking movepool causes it to require some support until all its good moves become available later on.
Weak to: Electric, Grass, Ice, Fairy, and Bug
Immune to: Psychic
Resistant to: Water, Fire, Ice, Dark, Steel, and Ghost
Venusaur @ Venusaurite
Availability: Lumiose City (Gift from Professor Sycamore)
Ability: Overgrow -> Thick Fat
Venusaur is designed to be a Special Tank, with middling stats save for its Special Attack and Special Defense. As Mega Venusaur, everything, except its HP and Speed, gets a major boost, allowing it to hit hard both physically and specially, as well as becoming even more of a tank. Grass/Poison is solid defensive typing in game, giving Venusaur several key resistances to Fighting-, Fairy-, and Water-types, at the cost of being weak to only two types, being Psychic and Flying. Mega Venusaur also gains the Thick Fat ability when it Mega Evolves, making it take neutral damage from Fire- and Ice-type attacks. Energy Ball is your Grass STAB move. Sludge Bomb is your Poison STAB move, which kills Fairy types. Leech Seed is there so you can sap your opponent's health while attacking them. Coverage for Venusaur would be Earthquake to cover from its Fire weakness. In Major Battles, Bulbasaur sits out from Grant and Olympia, but otherwise does well in all other battles. Just don't put it up against Ramos's Jumpluff, Valerie's Mr. Mime, or Lysandre's Honchkrow. It also performs outstandingly against all of the Elite Four, except for Drasna. Overall, Venusaur more than earns its place as the best Grass-type in the game. The only drawback is its relatively poor movepool, and even then it gets what it needs, when it needs it.
Weak to: Fire, Ice, Psychic, and Flying
Resistant to: Fighting, Water, Grass, Electric, Fairy
Availability: Route 6 (15% encounter rate)
Ability: Stance Change
Honedge and Doublade's stats favor their physical attack and defense while Aegislash's stats are much more balanced so it is very bulky in Shield forme and can hit very hard in Blade forme. They are all quite slow though. It's typing, is very good defensively. Ghost gives an immunity to the common Normal and Fighting types early on, while Steel gives an immunity to Poison and resistance to many other types. Offensively it comes with great coverage. Iron Head is your Steel STAB move, as it destroys Fairy types. Shadow Sneak is your Ghost STAB move, even though it may be weak out of the other moves, like Shadow Ball and Shadow Claw, but since Aegislash is fairly slow, it gives you priority. Coverage for Aegislash are Sacred Sword for Normal and Dark types. And King's Shield is there so you can protect yourself from opponents moves, and if they use contact moves, then their attack stat is lowered by 2. And for it's major battles, Honedge beats Grant's Amaura, Korrina, Ramos, Valerie (outside of Mawile), Olympia (beware Meowstic), and Wulfric. InFlyi the Elite Four don't even bother with Malva, but it can contribute against the others. Against Diantha, you can set up on Hawlucha and sweep her whole team, easily. Overall, Honedge's defensive typing and great stats allows it to be a strong asset for the whole story. You need to find a Dusk Stone to get Aegislash, which is at Terminus cave, and it's so late in the story. However, Doublade is a very capable Pokemon—especially with Eviolite—so if you plan on evolving it, wait until you get the Dusk Stone.
Weak to: Fire, Ground, Ghost, and Dark
Immune to: Poison, Fighting, and Normal
Resistant to: Flying, Rock, Bug, Steel, Grass, Psychic, Ice, Dragon, and Fairy
Aerodactyl @ Aerodactylite
Availability: Ambrette Town (Gift from Old Amber in Glittering Cave, need Rock Smash)
Ability: Pressure -> Tough Claws
Your team's flying type. Aerodactyl is very fast and has a fairly high Attack stat. It also has decent defenses to couple with its resistances. As Mega Aerodactyl, it becomes even more faster, much more powerful, getting the ability, Tough Claws, which boost your contact moves by 30%, as well as improved bulk. The Rock/Flying-typing gives Aerodactyl some nifty, relevant resistances though weaknesses to Water-, Rock-, Electric-, Steel-, and Ice-types hurt. Offensively, its typing is excellent, especially after investing in coverage. Stone Edge is your Rock STAB move, as it may not be boost by Tough Claws, since it's not a contact move, but it does help with its Ice weakness. Fly is your Flying STAB move, which not only it's pretty useful in-game, so you won't have to go to the PC box and get an HM slave for Fly, but it also gets boosted by Tough Claws. Coverage for Aerodactyl would be Thunder Fang for those pesky Water-types, as it gets boosted by Tough Claws. And Earthquake to cover it's Rock, Electric, and Steel weakness. Even if you have Bulldoze or not, Grant will be a hard challenge for Aerodactyl. After Grant, Aerodactyl has the typing and movepool to dominate nearly everything you encounter. For major battles, watch out for Clemont's Magneton and Lysandre's Mega Gyarados. Oh yeah, don't forget to pick up the Aerodactylite in Ambrette Town as it is crucial to Aerodactyl's success.
Weak to: Water, Rock, Steel, Electric, and Ice
Immune to: Ground
Resistant to: Normal, Flying, Poison, Bug, and Fire
Availability: Route 9 (20% encounter rate)
Ability: Dry Skin
Helioptile has great Speed and Special Attack and decent Special Defense. Its HP and Defense are quite poor however, meaning it cannot take physical hits well, and its Attack is too low to warrant use. Electric/Normal is an odd typing. Gives it an immunity to Ghost, resists Electric-, Flying-, and Steel-types, and is only weak to Fighting- and Ground-types. Offensively, Electric is decent, but Normal doesn't really compliment it well. Thunderbolt is your Electric STAB move. Razor Wind is your Normal STAB move. Coverage for Heliolisk, is Surf for its Ground weakness. And Focus Blast serves as a nifty coverage to Dark, Steel, Rock, Normal, and Ice types. Major Battle wise, Siebold is a cakewalk for Heliolisk, and Lysandre is also pretty easy (be careful against Mienshao though). Keep it far away from Korrina (and maybe it can destroy Hawlucha) and Drasna is tough for it. Otherwise it has a decent matchup record though its frailty as a Helioptile lets it down. Overall, be sure to trade the Intriguing Stone to the Hiker in Shalour City for a Sun Stone to evolve as early as possible. It doesn't need any level-up moves anyways. Dry Skin is preferred as an ability for Heliolisk, because the Water immunity is quite handy. Helioptile is a decent choice, but only once it evolves.
Weak to: Fighting and Ground
Immune to: Water(If it has Dry Skin) and Ghost
Resistant to: Flying, Steel, and Electric
Garchomp @ Garchompite
Availability: Route 13 (20% encounter rate)
Ability: Sand Veil -> Sand Force
-Crunch/Stone Edge/Poison Jab
Gible's stats lend itself towards a powerful, bulky physical attacker with enough speed to outrun most of the game as a Garchomp. The Dragon/Ground-typing lends itself to a lot of good matchups against major battles with a few bad ones, as well as offering good offensive coverage and plenty of resistances. Dragon Claw is your main Dragon STAB move. Earthquake is your main Ground STAB move (upgraded from Bulldoze), It has good coverage, like Flamethrower provides a decent special option if needed for Ice types. You can either run Crunch, which is coverage for Ghost and Psychic types, Stone Edge which is coverage for Flying, Bug, and Fire types, or even Poison Jab provides coverage for Fairies . Gible boasts a strong matchup against Clemont, most of Team Flare, and most of the Pokémon League, as well as ripping through Serena/Calem's last fight. It is also able to chip in against Valerie and Wulfric despite an overall terrible matchup. Gabite really appreciates an Eviolite as it greatly improves its ability to take on several opponents with its increased bulk and greatly reducing any dud periods it would have. Its arrival in Route 13 (between Lv26 and Lv28) may necessitate some grinding to get it up to par upon capture. The Garchompite is post-game, btw.
Weak to: Ice, Dragon, and Fairy
Immune to: Electric
Resistant to: Poison, Fire, and Rock