You didn't hesitate on making this matchup, huh?
Zero has defeated X in the past and is thought to be more skilled. In base, X only ever defeated Zero once with the aid of Rocket Racoon. Keep in mind, Zero was overwritten by Grandmaster Meio at the time, so its arguable if Zero put all his skill in that battle. So just going off their base selves, no amps, Zero should logically win
But that's where this gets interesting. X can amp himself with the Armor of Light while Zero has X-Factor. X can resist fire-based attacks while Zero can resist ice-based attacks. X has more versatility and spread for his attacks, but Zero has a better range and can increase the amount of attacks he uses at once with Sougenmu. So how do we get a tie breaker from all this?
Well, aside from Zero having better Dash Thrusters mobility, his analytical capabilities are much higher, as he can analyze his opponent's moves and data in a glance. X and Zero would be advanced enough to analyze each other, but Zero has the advantage of having actually fought and defeated X before. Its just more of a fair fight in this case since the Armor of Light and X-Factor are being taken into account here
Overall, X puts up a better fight than he did when he last fought Zero one-on-one, but loses to his friend and rival IMO due to prior knowledge and analytical capabilities
Voting for Zero