Note: This one will focus mainly on why Charizard isn't S like people have suggested. If his Mega Forms were not so powerful; especially late; I'd even have suggested B. Also, remember this comes at the cost of Venusaur and Blastoise.
Charmander/Charmelon/Charizard/Charizard X: - A Tier
Charmander/Charmeleon/Charizard/Charizard Y: - A Tier
Availability: Gift from Professor Sycamore
Stats:
Charmander: 39/52/43/60/50/65
Charmeleon: 58/64/58/80/65/80
Charizard: 78/84/78/109/85/100
[Estimated]
MegaCharizard X: 78/130/111/130/85/100
MegaCharizard Y: 78/104/78/159/115/100
Charizard is a fast special attacker with acceptable bulk and physical attack. MegaCharizardX changes into a mixed attacker, with good physical bulk, while MegaCharizardY focuses more on the special side, both on offense and defense.
Typing: Pure Fire is a pretty good typing, both on the offense, and above average on defense with the addition of Fairy. Fire/Flying is actually somewhat redundant on the offense, only gaining coverage on Fighting-types, and some navigation around types Charizard shouldn't be fighting in the first place, and arguably is worse than pure fire on defense, due to Thunderbolts and random Rock moves being a lot more common than random ground attacks. Fire/Dragon is exceptional offensively; and solid on the defense, although being weak to Rock, Ground, Dragon and Fairy is pretty dangerous.
Movepool: By level-up, Charizard's movepool is pretty poor; bordering on awful. Until Lv 28, Charmeleon has to make do with Ember and Dragon Rage; which doesn't scratch fairies and quickly falls off, especially with evolved wilds. [Lv 17 is pushing it for OHKO'ing anything with Dragon Rage]. At Lv 28, it learns Fire Fang, which is not much better than Ember due to running off Charmeleon's lower Atk stat. It takes until Lv 32 for Charmeleon to learn an acceptable special STAB; in Flame Burst; at the same time Venusaur gets Petal Dance; and Blastoise would have Surf; making Charmeleon a lot weaker than his counterparts mid-game.
It takes until Lv 47 for Charizard to learn Flamethrower; after which there is nothing of note. However, Charizard has a good TM pool, and when reaching the move re-learner, has access to Flare Blitz; Heat Wave; Air Slash and Dragon Claw, a potential massive power spike for either Charizard lategame.
Major Battles:
Grant - Has minor use due to Dragon Rage.
Korrina - Poor. Hawlucha can use Rock Tomb [Although I'm not 100% if this one knows it; I recall several in-game which did], and Charmelon lacks any good attacks at this point, so gets out-muscled.
Ramos - Destroys the Gym.
Clemont - Poor; due to weakness to Electric. Charizard X fares better due to Dragon-typing; however.
Valerie - Charizard Y outperforms X here; but both are average at best due to movepool issues; despite resistance to Fairy.
Olympia - Average
Wulfric - Rips apart
Team Flare - Fairly bad; due to bad movepool most of the game, on top of coverage mainly being Shadow Claw and Fire/Normal until the final showdowns.
Rival - Acceptable. Great if they pack Chesnaught
Malva - Poor; Charizard Y is an outright handicap.
Wikstrom - Sweeps
Dransa - Charizard X gets killed. Charizard Y/Base can't do much
Seibold - Keep away from him. Charizard X may get some work done with Dragon Claws, however.
Diantha - Charizard Y can do major work here. Charizard X is weaker, however, and will get OHKO'ed by Mega Gardevoir.
Additional Comments: Taking Charizard likely means that you've not taken Fennekin; who is probobly the best Kalos starter. Charizard is very powerful lategame; but is awful in the midgame; and becomes heavily reliant on his Mega Evolution to keep up with what Blastoise and Venusaur can do with their much, much better STAB moves. Charizard realy shines in the lategame; however; when his movepool issues vanish thanks to the Move Re-learner. This is the main reason why Charizard is A Tier. It has a lot of power Mega-Evolved, but has to do so literally every battle; slowing things down a lot. Plus Y's Drought animation; and the fact X will mainly be using the 2-turn Fly or the weak Fire Fang for the Tough Claws bonus most of the game.
Honestly if you could skip Mega Evolution's animation, I might consider putting him at S simply for the sheer stats of the MegaEvos addressing movepool issues without slowing down combat; but that's not the case.
As a note; Charizard without his Mega Evolutions would be B-Tier. He is a
lot better for them; as they address his midgame movepool. Seriously; his movepool is just awful in the midgame.