It would be between Mega Charizard X & Dragonite
The only way Charizard could beat Dragonite would be to set up a Dragon Dance, but Dragonite can do the same thing. Dragonite also has the ability Multiscale so any damage from attacks that hit him at full hp is cut in half. but fukk all that. lets get some math up in here
252+ Atk Charizard X Outrage vs. 4 HP / 0 Def Multiscale Dragonite: 298-352 (91.9 - 107) 56.5% chance to OHKO.
so charizard could POSSIBLY ohko without a dance it seems, RNG could screw you at anytime. And this is a Dragonite set that doesnt run defense. Weakness Policy Dragonite with Defense investment is becoming quite popular lately. And Choice Scarf Dragonite is always viable to out speed half the entire metagame.
what about Dragonite?
252+ Atk Dragonite Outrage vs. 4 HP / 0 Def Mega Charizard X: 404-476 (135.5 - 159.7%) -- guaranteed OHKO
Dragonite is guaranteed the KO everytime.
So that it seems that both can OHKO, it would come down to who's faster
Dragonite is Base 80, Charizard is Base 100.
Charizard is going to hit first (If Dragonite is not scarfed), if he gets RNG screwed, Dragonite is going to kill him guarantee'd.
So its up to luck tbh.
Im still riding with Dragonite though, Hes been top dog (inb4 Salamence) for 5 Generations
Calc Source:
http://www.gamefaqs.com/boards/696959-pokemon-x/68383744