samurais were mostly nobles and politicians. They used bows and crossbows. it was only after the advent of guns when the bow was made obsolete that samurai started talking about thier bitchin samurai swords that they totally used to behead a thousand men in a single night.
If it's far away, I'd say samurai. Up close centurion all the way

A samurai would probably win against a legionary. Thing is, a legoinary is the basic unit and in japanese armies, ashigaru's were the basic unit and they were basically garbage (most of times). Legions work altogether, in formation and samurais don't. A samurai sword won't cut through a tetsudo.

That’s it. I’m sick of all this “Naginatas” bullshit that’s going on in the d20 system right now. Gladius deserve much better than that. Much, much better than that.
I should know what I’m talking about. I myself commissioned a genuine Gladius in Rome in the 3rd Century for 314612 Denarii (that’s about 400.000 Antoninianus) and have been practicing with it for almost 2 years now. I can even cut slabs of solid steel with my Gladius.
Roman smiths spend years working on a single Gladius and stab up to a million Germanic Savages to produce the finest blades known to mankind.
Gladiuses are thrice as sharp as Japanese katanas and thrice as hard for that matter too. Anything a katana can cut through, a Gladius can cut through better. I’m pretty sure a Gladius could easily bisect a Samurai wearing his very honorable armour.
Ever wonder why Japan never bothered conquering Rome? That’s right, they were too scared to fight the disciplined Roman army and their Gladiuses of destruction. Even in World War II, American soldiers targeted the men with the Gladiuses first because their killing power was feared and respected.
So what am I saying? Gladiuses are simply the best sword that the world has ever seen, and thus, require better stats in the d20 system. Here is the stat block I propose for the Gladius:
(One-Handed Exotic Weapon) 1d12 Damage 19-20 x4 Crit +2 to hit and damage Counts as Masterwork
(Two-Handed Exotic Weapon) 2d10 Damage 17-20 x4 Crit +5 to hit and damage Counts as Masterwork
Now that seems a lot more representative of the cutting power of the best Roman weapon in existence, don’t you think?
tl;dr = The Gladius need to do more damage in d20, see my new stat block.

Eh, no. Romans had shields, which the japanese never utilized since they had such limited resources. This means that in a one-on-one fight, the samurai has no defensive options, and his weak, often poorly made sword will be easily deflected, followed by a killing blow. Romans also had an institutional training program for their soldiers, with military philosophers and generals specifically working on ways for better combat, but the military program in Japan was largely unorganized and not designed for a sustained military force until far closer to the modern day. This is not to mention the fact that all Roman soldiers had armor, and upper class and older Romans had fucking impressive armor, that was well reinforced with leather and chainmail. Not to mention the gladius itself, which was well-designed, with good iron and reinforcement on the blade. It wouldn't just withstand a blow from a katana or naginata, it would probably bend the thing or even shatter it.

In conclusion: better training, better weapons, and better armor. The battle will always go to the Roman.

Even well made Japanese swords were made for slashing not for hacking. Due to sharpness it would also easily dull while hacking or slashing at a simple chainmail or plate armour. Only chance samurai would have versus armored enemy is to go for sockets or other holes. But then again, rapier is better for that. If by some miracle Japanese warriors carried plate armour as well they would still loose since regular swords were made to hack at the armour but katanas weren't. Only way for Japanese to win with katanas vs Romans would be in a butt naked/no armoured fight since 'regular' swords were harder to swing around.

Even then, Rome had, like I said, an institutional training program. Roman soldiers would spend probably full years of manhours over the course of their lives training in the basilicae to master melee and ranged combat in pretty much all situations. Even in a fight butt-naked with weapons, a roman soldier would still have been specifically trained to use his weapon as a way to get past an enemy soldier's without using either his armor or shield.

Most samurai were drunkards, gamblers, scholars, impoverished sell-swords who would resort to selling their daughters into a brothel - a well trained Roman soldier likely has the ability to outclass all but the upper echelons on the samurai class, but wven then superior armour would make up for any parity in skill (or gap in skill).

A lot of what is thought of when considering the samurai is a romanticised myth created by the Tokugawa in order to make reality conform to delusion; the samurai were meant to be upper class, but they were extremely impoverished, so the bakufu went out of its way to make their poverty a percieved "choice" and added a bunch of bullshit like loyalty and piety around their poverty in order to give some sort of mythical significance to being poor.

I'd say at least 7/10 the Roman is taking down the Samurai, depending on the Roman, the Samurai, and the timeframe - in a full war, Romans all the way, their seige machines would completely fuck their Japanese counterparts.

Also, most samurai used bows and spears because they were not particularly good at sword fighting. At most, the sword was for show or if they were challenged to a duel, in which case it would likely not be to the death. Somewhat correct me if I'm wrong.

But every Roman soldier got Gladiator training, so they were also well versed in one-on-one fights. Centurions were officers commanding 100 soldiers, so they were already well seasoned, larger and stronger than the average Roman soldier.
The Roman Centurion is also considerably better equiped with heavy armor, a huge shield and both spear and gladius. I'd think the Centurion would use his large shield and stature to try and knock the samurai down and then just stab or slash him with the sword. Even if it came down to stamina, a roman soldier was better trained for long marches loaded with their equipment.

It'd be an interesting fight nonetheless. But i'd say the Centurion wins both one-on-one and in group.

Well shit, out of shit soil you can only make shit resources.

better resources. Specifically human recources.

Romans had better armour and weapons though.

Better looking than every ass on the page.