Deathzilla @ Charizardite X
Ability: Blaze -> Tough Claws
EVs(Jolly): 252 Attack; 184 Speed; 72 HP
EVs(Adamant): 252 Attack; 200 Speed; 56 HP
- Dragon Dance
- Dragon Claw/Outrage
- Flare Blitz
- Roost/ Earthquake/ Brick Break
Role: Wall Breaker, Physical Sweeper, Late Game Cleaner
What it Does: Charizard X kills things, plain and simple. It has fantastic typing, with its only weaknesses coming from Dragon, Rock, and Ground, and no double weaknesses. Thanks to its impressive bulk, he can run many different sets and in all cases, has the ability to set up and sweep. Fire typing makes him immune to burn conditions, which makes him incredibly strong as an offensive physical pokemon. The speed investment set listed allows Charizard to outspeed base 91 and below enemies at +1 when he is at +1 himself. The most commonly listed matchup is Landorus-T holding a Choice Scarf, which gives him an offensive edge. Dragon Claw is your primary STAB, with 160 BP after its boost from Tough Claws it can basically destroy everything. For things that Dragon Claw either doesn't hit, or in the case that you run into a wall, Flare Blitz boasts 240 BP and annihilates almost anything that doesn't resist it. The other set is designed for raw power and damage, and the EVs allow him to (while at +1) out speed base 145 mons with beneficial natures like the new Mega Beedril running Dril Run or Mega Sceptile running Earthquake. The last move slot varies depending on preference and how much you like to believe what you read on the internet. Roost gives you the ability to heal up damage you either take by Flare Blitz recoil or by chip damage from hazards and enemies who manage to get in some priority hits or something. Generally, it is considered the best option. If you're like me, and you think wasting a turn healing up damage you do to yourself so you can do it again next turn is a bad idea, you can run a third attack for coverage. The two I have listed are good for general coverage since fighting and ground outside of fire have the best offensive coverage in the game, but there are other options if you fear specific enemies like Thunder Punch and Iron Tail.
Good Teammates: The best teammate that any Charizard X could ask for is a Scarfed Excadril. Before mega evolving, Zard takes 50% damage from Stealth rock. Fuck that shit. Excadril hits a lot of Charizard's counters and it has rapid spin making him freaking awesome. Any kind of hazard control is needed to keep Zard X safe to switch in. Greninja can serve as a special complement to an extent, as well as something like Thundurous slowing things down with Thunder Wave support (especially with the slower Adamant build). Strong special attackers to handle physical walls that somehow Charizard can't handle can help him out a lot.
What Counters it: For as awesome as he is, Zard X does have a few strong counters. First one, and the bitchiest to deal with is Azumarril. Water/Fairy is the polar opposite of Fire/Dragon, and it shows. Zard can't hit him for anything substantial unless you run Thunderpunch, and running tPunch just for Azu is a waste of a move slot. Mega Altaria is also a scary new counter who resists both STABs and can come back with a STAB of its own to kill you. Heatran absorbs fire and doesn't care too much about dragon claw. Diancie is the newest threat, as a Rock/Fairy it is also terrifying to deal with. Choice Scarf Garchomp is less common nowadays, but unless you can get yourself up to +2 speed, there is nothing you can do about that 102 base speed. Without a good coverage move and/or support from the rest of your team these threats will stop you from succeeding.
Any Additional Info: With the exception of maybe Mega Alteria, Mega Charizard X is the best Dragon Dancer in OU. This is only one iteration of builds for Charizard X, and there are many different EV spreads and move sets that can change the way he is played and the kind of support he needs.