Charizard is one of the few pokemon with multiple Mega evos.  First I’ll cover what each version of Charizard brings to the table.  

So your standard non-mega Charizard, at first glance, looks like a pretty... weak special sweeper.  109 base SpA and 100 base Speed.  The stats aren’t what strike me as “weak” though.  Typing, Fire/Flying, isn’t great.  4x weakness to rock means Stealth Rocks cut your HP in half on each switch in.  

Mega X is next.  Horray he’s a Yellow and gold Black and Blue Dragon type now!  The typing DOES make him better.  Yeah he gains a weakness to Dragon and Fairy but he doesn’t get totally destroyed by rock types now.  On top of that he now gets to add another STAB move to his move pool with Dragon Pulse or Dragon Claw.  What I’d love to note about this is his ability and stat spread.  Tough Claws makes his 130 base attack very powerful with the use of things like Dragon Claw and Flare Blitz.  Now let’s not forget that he can get access to Belly drum or Dragon Dance for even more sweeping power.  But just because his ability only powers up Contact moves doesn’t mean he can’t use a special attack or two.  

Mega Y finally.  Alright so I looked at the sprite and he looked pretty cool.  But I kinda came when I saw his 159 SpA Stat.  Yes, yes, his typing still sucks with Fire/flying.  However that 159 SpA stat is HUGE.  Drought also brings out a lovely move called “Solar Beam” and makes it good for sun teams.  

Alright so.  Regardless on which set you run, unless it’s not mega-stone, I’d run a +speed nature.  Timid or Jolly generally.  But I’m a fan of Mega Y for a special sweeper and Mega X for a physical.

Mega Y Mega Y stone Timid HP/SpA/Spe 4/252/252 Flamethrower/Fire Blast Solarbeam Dragon Pulse Hidden Power (whatever you decide really.)/Air Slash

Mega X Mega X stone Jolly HP/Atk/Spe 4/252/252 Dragon Claw/Outrage Flare Blitz/Fire Fang Dragon Dance/Swords Dance Roost/Earthquake

Charizard Life Orb Solar Power Timid Hp/SpA/Spe 4/252/252 Flamethrower/Fire Blast Dragon Pulse Hidden power (again whatever helps your team out) Air Slash

So, ever since the beginning of the game we’ve had this infamous thing called a critical hit.  In gen 1 it was based entirely on your speed (and base crit ratio).  Causing Persian to have roughly a 96% crit rate with slash.  However, generation 2 and onward we got a new way to deal with crit.  It’s called a “Domain”.  Domain goes from 1 to 5 going up in crit chance each level.  Every pokemon has base D1.  Moves like Focus Energy and abilities like Super Luck and items like Scope lense raise a pokemons Domain.  

D1 = 6.25%

D2 = 12.5%

D3 = 25%

D4 = 33%

Ice shard is essentially its bread and butter.  Very nice priority stab move.  Next up would probably be Earthquake.  Pretty generic move that, if it learns it, you should generally teach it.  Regardless of pokemon.  You have a little freedom with the next move.  You could go with either Rock Slide or Stone edge.  This is entire based on how you value accuracy and secondary effects.  personally I'd go with Stone Edge because who doesn't love a 25% crit chance.  Sadly its movepool is a little lack luster and I've only used it on one team.  But you can fill the last move in with Stealth rocks/Super Power/Iron Head.  Item could vary between Scarf/Sash/Band

Personally I’d stick with

Mamoswine @Focus Sash Adamant 4/252/252 Hp/Atk/Spe Ice Shard Stealth Rocks Earthquake Stone Edge