So first off, I'll write up two that I did not use all the way. They're a little theory-monish, but I think I got a good enough grasp on how they work.
Chesnaught / Chespin
Dunsparce
The rest were carried through the entirety of the game.
Vivillon / Scatterbug
/
Charizard [X] / Charmander
Ok, I'll write the next 4 soonish.
Xerneas, Aurorus, Doublade (I never got Aegislash haha), and Goodra.

Chesnaught / Chespin
Chesnaught - B/C Tier
Availability: Starter Pokemon, so its availability is perfect.
Stats: All of its forms have a great Defense stat as well as Attack. Its Speed is very underwhelming however, and it will not be outspeeding a lot of Pokemon at your level. Be ready to take some hits.
Typing: Its STAB typing is pretty nice, hitting 7 types super effectively. However, Grass/Fighting is not as good defensively. Chesnaught has 6 weaknesses, including a 4x weakness to the common flying type. 6 resistances are nice though.
Movepool: This is what stopped Chesnaught from being great for me. In its level up movepool, Chesnaught gets a wonderful progression of grass-type moves. However, its fighting type learnset is essentially nonexistent. Mine knew Power-Up Punch for the majority of its tenure. Power-Up Punch is a respectable move, but it, like a lot of Chesnaught's level up moves, is geared towards a playstyle that is not efficient for a simple in-game run. Leech Seed, Spiky Shield, and Bulk Up are inefficient for a playthrough. Once you do get powerful STAB moves, you are in the 50s and 60s level-wise and the two moves (Wood Hammer and Hammer Arm) have negative effects for the user. Even the Brick Break TM is pretty late in the story.
Also - Mine had Pin Missile as well as STAB - That 3 move combo is resisted by Poison, Flying, and a pretty large number of Dual-Types. And until late game, this and weak 60BP or less coverage moves are what you'll be relying on.
Major Battles: Chesnaught is pretty mediocre regarding gyms. You are disadvantaged in the 1st, and the Rock leader uses Pokemon that are rather unforgiving towards Grass types. Once you gain Fighting type, you realize that you are disadvantageous against the last 3 gyms. It does well against Fighting (minus Hawlucha) and Electric, but the Grass Gym resists your STAB nearly completely and packs advantageous secondary types. Team Flare carries a lot of Fire, Poison, and Flying types; perfect for resisting Chesnaught and hitting with SE. They carry Dark types, but the common Houndour is also fire type. I stopped using him before the E4, but it seems as though it would be alright. You have SE STAB on 2 members, but both have Pokemon to hit you back with SE secondary types.
Additional Comments: Chesnaught does pretty well in normal encounters, but it was just underwhelming. You hit a good number of Pokemon SE, but there are just as many that you can barely get neutral on.
Availability: Starter Pokemon, so its availability is perfect.
Stats: All of its forms have a great Defense stat as well as Attack. Its Speed is very underwhelming however, and it will not be outspeeding a lot of Pokemon at your level. Be ready to take some hits.
Typing: Its STAB typing is pretty nice, hitting 7 types super effectively. However, Grass/Fighting is not as good defensively. Chesnaught has 6 weaknesses, including a 4x weakness to the common flying type. 6 resistances are nice though.
Movepool: This is what stopped Chesnaught from being great for me. In its level up movepool, Chesnaught gets a wonderful progression of grass-type moves. However, its fighting type learnset is essentially nonexistent. Mine knew Power-Up Punch for the majority of its tenure. Power-Up Punch is a respectable move, but it, like a lot of Chesnaught's level up moves, is geared towards a playstyle that is not efficient for a simple in-game run. Leech Seed, Spiky Shield, and Bulk Up are inefficient for a playthrough. Once you do get powerful STAB moves, you are in the 50s and 60s level-wise and the two moves (Wood Hammer and Hammer Arm) have negative effects for the user. Even the Brick Break TM is pretty late in the story.
Also - Mine had Pin Missile as well as STAB - That 3 move combo is resisted by Poison, Flying, and a pretty large number of Dual-Types. And until late game, this and weak 60BP or less coverage moves are what you'll be relying on.
Major Battles: Chesnaught is pretty mediocre regarding gyms. You are disadvantaged in the 1st, and the Rock leader uses Pokemon that are rather unforgiving towards Grass types. Once you gain Fighting type, you realize that you are disadvantageous against the last 3 gyms. It does well against Fighting (minus Hawlucha) and Electric, but the Grass Gym resists your STAB nearly completely and packs advantageous secondary types. Team Flare carries a lot of Fire, Poison, and Flying types; perfect for resisting Chesnaught and hitting with SE. They carry Dark types, but the common Houndour is also fire type. I stopped using him before the E4, but it seems as though it would be alright. You have SE STAB on 2 members, but both have Pokemon to hit you back with SE secondary types.
Additional Comments: Chesnaught does pretty well in normal encounters, but it was just underwhelming. You hit a good number of Pokemon SE, but there are just as many that you can barely get neutral on.

Dunsparce
Dunsparce - C/D Tier
Availability: Dunsparce is available very early, before the first gym. It's a little rare though.
Stats: Dunsparce's Stats are amazing at the stage you get it - a solid 70 Attack with 100/70/65 Defenses. These remain good until everything around you evolves while Dunsparce... doesn't. Its stats are awful towards mid and late game.
Typing: Normal doesn't get any SE hits, but you do get the very useful STAB on the early Return TM.
Movepool: Before the first gym, it has just enough to get by. Defense Curl and Rollout can take out the first gym while taking very little damage. You get Return shortly after that win, and that will carry Dunsparce until you inevitably box it. Dunsparce's Level-Up and TM learnset make it into a Jack of All Trades, Master of None. Dunsparce gets a lot of cool tricks that it doesn't have the stats to back up. Yawn, Roost, Coil, Flamethrower, Ice Beam, Thunderbolt, Poison Jab, and Drill Run are just a few interesting moves you can learn. Dunsparce has a great movepool, it just doesn't have stats or typing to supplement it.
Major Battles: As mentioned before, the 1st Gym is very easy. The second and third are not kind to Dunsparce. The rest are neutral and Dunsparce can get SE coverage moves for most, but by then Dunsparce will begin to fall behind and the advantages won't be enough to keep Dunsparce afloat. Rollout is very nice for the first few Flare encounters, as it's super effective on Fire, Bug, and Flying.
Additional Comments: Dunsparce is stellar early game. This is why I'd possible give it C rank. However, that doesn't make up for how awful it is later on. I was very excited when I saw it early on because its my favorite Pokemon, but he once again proves that he can't keep up with... well, anything that evolves really.
Availability: Dunsparce is available very early, before the first gym. It's a little rare though.
Stats: Dunsparce's Stats are amazing at the stage you get it - a solid 70 Attack with 100/70/65 Defenses. These remain good until everything around you evolves while Dunsparce... doesn't. Its stats are awful towards mid and late game.
Typing: Normal doesn't get any SE hits, but you do get the very useful STAB on the early Return TM.
Movepool: Before the first gym, it has just enough to get by. Defense Curl and Rollout can take out the first gym while taking very little damage. You get Return shortly after that win, and that will carry Dunsparce until you inevitably box it. Dunsparce's Level-Up and TM learnset make it into a Jack of All Trades, Master of None. Dunsparce gets a lot of cool tricks that it doesn't have the stats to back up. Yawn, Roost, Coil, Flamethrower, Ice Beam, Thunderbolt, Poison Jab, and Drill Run are just a few interesting moves you can learn. Dunsparce has a great movepool, it just doesn't have stats or typing to supplement it.
Major Battles: As mentioned before, the 1st Gym is very easy. The second and third are not kind to Dunsparce. The rest are neutral and Dunsparce can get SE coverage moves for most, but by then Dunsparce will begin to fall behind and the advantages won't be enough to keep Dunsparce afloat. Rollout is very nice for the first few Flare encounters, as it's super effective on Fire, Bug, and Flying.
Additional Comments: Dunsparce is stellar early game. This is why I'd possible give it C rank. However, that doesn't make up for how awful it is later on. I was very excited when I saw it early on because its my favorite Pokemon, but he once again proves that he can't keep up with... well, anything that evolves really.
The rest were carried through the entirety of the game.

Vivillon / Scatterbug
Vivillon - A
Availability: Scatterbug is common before the first gym.
Stats: Vivillon gets 90 Special Attack and Speed at level 12, which is pretty incredible. The rest of its stats are not great and this becomes more noticeable later in the game but it is not a big problem.
Typing: Bug/Flying is a generic typing. 5 Weaknesses, 3 Resistances and 1 helpful Immunity. STAB gets 5 SE hits.
Movepool: This is where Vivillon shines. You get Struggle Bug right away, which is now 50BP and still lowers Special Attack. You also get Psybeam very soon, which is a very usable coverage move. Just as Struggle Bug feels a little weak, you get Bug Buzz to plow through everything. Towards the late-mid game, Vivillon's stats start to feel low... until you get Hurricane and Quiver Dance. Compoundeyes Hurricane is hilariously powerful and Quiver Dance makes it even more ridiculous. You also get a few auxiliary moves like Aromatherapy and the new Powder, but Hurricane is all that you really need.
Major Battles: Vivillon fairs decently against the gyms. 1, 2, 5, and 6 are bad matchups but the others are fairly easy for the butterfly. It can beat Team Flare's Dark and Poison Types, but the abundant Fire and Flying are tough.
Additional Comments: Compoundeyes is crazy. I imagine that a Shield Dust one wouldn't come close to A tier.
Availability: Scatterbug is common before the first gym.
Stats: Vivillon gets 90 Special Attack and Speed at level 12, which is pretty incredible. The rest of its stats are not great and this becomes more noticeable later in the game but it is not a big problem.
Typing: Bug/Flying is a generic typing. 5 Weaknesses, 3 Resistances and 1 helpful Immunity. STAB gets 5 SE hits.
Movepool: This is where Vivillon shines. You get Struggle Bug right away, which is now 50BP and still lowers Special Attack. You also get Psybeam very soon, which is a very usable coverage move. Just as Struggle Bug feels a little weak, you get Bug Buzz to plow through everything. Towards the late-mid game, Vivillon's stats start to feel low... until you get Hurricane and Quiver Dance. Compoundeyes Hurricane is hilariously powerful and Quiver Dance makes it even more ridiculous. You also get a few auxiliary moves like Aromatherapy and the new Powder, but Hurricane is all that you really need.
Major Battles: Vivillon fairs decently against the gyms. 1, 2, 5, and 6 are bad matchups but the others are fairly easy for the butterfly. It can beat Team Flare's Dark and Poison Types, but the abundant Fire and Flying are tough.
Additional Comments: Compoundeyes is crazy. I imagine that a Shield Dust one wouldn't come close to A tier.


Charizard [X] / Charmander
Charizard w/ Mega X - S
Availability: Given to you from Sycamore pretty early on.
Stats: Charmander and Charmeleon are fast and have good offensive stats. Same with Charizard. Charizard X takes it up to 11. Both Attacks are 130, and its ability improves the damage of contact moves. Its defenses also become sturdier.
Typing: Fire/Flying has 3 common weaknesses, but 6 common resistances and an immunity. You can also change to fire/dragon midbattle, where you lose your immunity but lose your common Water and Electric weaknesses and get the very rare Dragon.
Movepool: You get Fire Fang and Flame Burst shortly before evolving and those will carry you while you are in X form. While you don't learn any useful non-fire moves level-up, Dragon Claw, Earthquake, and Brick Break will raise hell in Mega Form.
Major Battles: Charizard X does very well against nearly every major battle. The only ones with really negative matchups are the 2nd gym and the Water type E4 member. Charizard has enough offensive presence that neutral hits are enough to score a lot of OHKOs.
Additional Comments: Seriously just press Mega Evolve and laugh because you know you're going to win.
Availability: Given to you from Sycamore pretty early on.
Stats: Charmander and Charmeleon are fast and have good offensive stats. Same with Charizard. Charizard X takes it up to 11. Both Attacks are 130, and its ability improves the damage of contact moves. Its defenses also become sturdier.
Typing: Fire/Flying has 3 common weaknesses, but 6 common resistances and an immunity. You can also change to fire/dragon midbattle, where you lose your immunity but lose your common Water and Electric weaknesses and get the very rare Dragon.
Movepool: You get Fire Fang and Flame Burst shortly before evolving and those will carry you while you are in X form. While you don't learn any useful non-fire moves level-up, Dragon Claw, Earthquake, and Brick Break will raise hell in Mega Form.
Major Battles: Charizard X does very well against nearly every major battle. The only ones with really negative matchups are the 2nd gym and the Water type E4 member. Charizard has enough offensive presence that neutral hits are enough to score a lot of OHKOs.
Additional Comments: Seriously just press Mega Evolve and laugh because you know you're going to win.
Ok, I'll write the next 4 soonish.
Xerneas, Aurorus, Doublade (I never got Aegislash haha), and Goodra.