I'm going to assume that the premises is that it's seen from the perspective of a culture, meaning it encompasses a large group of people, and that we talk about real warriors, not soldiers.
What I would see her as the key factor is mentality as that's what sets it fundamentally apart from soldiers. A soldier and a warrior could be fighting for the same army, wear the same armour, but what sets the one apart from the other is his mentality. So what you search for is a culture that has a common warrior mentality.
Amazones:
You can scratch them as they're not real, they're a mythological race. Of course every myth has truth in them. It has been more or less accepted that the Amazon myth finds its origin in a race that lived somewhere north, north-east of the Black Sea. They had a culture where woman were highly valued and they could indeed also fight. But this was the same for the Vikings, woman were often also a part of their raiding parties and they fought alongside the men and Spartan woman also received battle training. There isn't anything that particularly makes the historical Amazones to be considered the fiercest, most of what we know, like that they cut off one of their breasts so that they could shoot better with bow and arrow, belongs in the realm of myths.
Gladiators:
These were pretty much sportsmen. Often slaves, but also people who volunteered, to fight each other for entertainment. The reality is that these fights were not as bloody as most people believe. In fact it was quite uncommon. It of course happened, but in the end it costs money to train these fighters properly as they needed to entertain the audience. If your fighters die all the time, you are not going to make a profit out of them. So death matches were more exception than rule. The things they wore were also meant to give an entertaining fight. So you can scratch them too.
Knights:
This is a tricky one. One of the persons I quoted below said that they range over a wide time period, but that's not really true. Yes there is an entire period of evolution that would eventually lead to the era of knights and that era would leave behind a long legacy, but that something one day would evolve into a knight or that something was derived from it, doesn't make them knights. I assume you are referring to the well known traditional image of a European knight, but that's the issue here. The image of the knight has been far more durable than the knight itself. You can still be knighted in the United Kingdom, but the reality is that the time of their prowess on the battlefield already was coming to an end around the year 1300. Their grand era was around 1000-1300.
As said it's mainly their image that survived as even after they lost their use on the battlefield, for centuries they kept up the image alive of the honorable knight with jousting tournaments and what not, but that's the point, it's just an image, a tradition, something prestigious. Even in the 19th century people were hosting jousting tournaments in the USA. But the truth is, as said, their practical use had ended long ago and even when they dominated the battlefield, you could hardly call them real warriors. It's true that they probably had the best equipment to survive a fight compared to the other groups, however that's exactly the problem here. There was an entire culture build around the chivalrous knight and that was also how wars were fought. Both parties were expected to honor the code of the knight and they all fought wars the same way. Not doing it like that would be considered not honorable. It pretty much came down to they sought a place that would be ideal for knights to fight, that they fought exactly the same and that the victor would respect the loser. You did not kill a knight, when a knight beat another knight, he was expected to spare his opponent and then his kin could pay to set the captured knight free. These captured knights were also treated like honorably guests, they sometimes even had to swear to not escape.
As much as I always loved knights, these were not really true warriors. They were also always mounted and with their heavy armour they were very restricted in how they could fight, they simply fought in a way that would suit the knight the best as both sides used knights. You can consider it almost to be a bit pompous. They were not so much warriors as people who wanted to show off their social standing and everything that comes along with it. It's not coincidence that the moment the knights left the battlefield, was the moment people started saying "screw that, we fight how we want", "taking prisoner? My pitchfork up your ass ya". Entire armies consisting out of knights have known to be butchered by mere peasant armies simply because they fought in a much more practical way. From that point on chivalrous knights was something for books and a manner to pass the time for the nobility. So you can scratch knights.
Samurai:
Samurai definitely is the group that follows the idea of 'honor' the most, but in the end they are pretty much the eastern equivalent of the western knights. They also follow a code of honor and this means mostly that they have to serve their lord and this does not include only fighting. When their honor is tarnished, they had to commit Seppuku. So in the end you can't consider them real warriors as that warrior mentality is missing. You can consider them more like elite soldiers. They have to defend and serve their lord, which includes fighting for him when they have to avenge him. You might seen the movie 47 ronin, but in the end it's all about the honor, fighting is just a means to defend it. It isn't a real fighting culture, so I wouldn't consider them the fiercest either. The mentality for it is missing.
Vikings:
No doubt they are fierce and they definitely had a warrior mentality. But still despite Vikings actually being not that badly equipped, in the end their success was more the result of the weakness of their opponents rather then their own doing. Most of the time Vikings attacked in small bands targeting cities, towns and monasteries that were had little to no protection whatsoever. At this point in time the European kings and lords actually had not much real power. Many of them only ruled in name over most parts of their territories. They did not have a central government, so in the end it were the local lords who in name of their king ruled, but in reality pretty much controlled these territories as they saw fit. So you had hundreds, even thousands of lords who controlled small pieces of land with no central governing institution above them. The power of the king was pretty hollow. With other words they were sitting ducks.
Yes after a while there have been larger groups of Vikings, almost armies, but that was still quite rare. Their specialty came pretty much down on hit and run, fast raids, too fast for anyone to do anything about it. But when kings and lords starting to organize their government and with the rise of cities, raids like these were pretty much impossible.
Vikings were fierce warriors, they definitely had that warrior mentality as if they fought and died well, they would be awarded in the next life. However they mainly were able to do the things they did due to the weaknesses of their opponents rather than just their own strength. On top of that you shouldn't forget that not all Vikings were raiders and fighters, many were explorers, traders and merchants.
Spartans:
The Spartans are the ones I would call the fiercest warriors as their entire culture breathed out fighting, both for men and women. Fighting was the life of a Spartan. Here is where they differ from Vikings. It's an entire culture, they don't have soldiers, they only have warriors that are highly disciplined, but remain having that fighters-mentality. Every single Spartan fighter was also very well equipped seeing the era they lived in. So they were simply far more larger than Vikings. It were first and foremost the Spartan who delivered heavy blows to the Persian Empire. I mean what was Sparta compared to the vast Persian Empire? And eventually the Spartans even succeeded in subduing Athens even on the seas while Athens was a naval power. Their mentality shows even in their architecture. Sparta never made large, fancy cities like Athens. There is barely any archeological evidence left of their existence and that's something the Spartans did on purpose. Well not the idea of not leaving anything physical behind, but simply that they do not need all of that as they are warriors. Fighting is what they do.
Logically if you pit a heavy armoured cavalry of knights against a Spartan army, the latter would likely be beaten, but then you are ripping them out of their historical context. I mean there is a reason as why 'Spartan' training is synonymous for brutal, extremely heavy training. I mean for a Spartan living a long live was considered an insult.
but seriously, the romans arent that much of big shots before a certain superhero whose last line was "et tu, brute?!!11!" appeared. my *****, carthage's hannibal barca literally ohko'd the roman army in italy, with an ingenious tactic.
alexander the great's phalanxes rekt the brunt of darius' (haha hes a loser) huge ass army.
the spartans too were defeated by a much smaller kingdom, by a prince (idr the little ****'s name) who made a minor adjustment to his army, by strengthening his right flank so as to render the spartan left useless.
medieval knights are total pricks, theoretically the amazons would be able to wreck them without even lifting a sword. all they have to do is stand in from thine good sirs who dont attack women.
gladiators, are uhm losers too. vikings are like ferocious af but their assaults are too rage driven and some planning from even a non caesar-led rome could finish them off so idk.
now look at storm troopers. unless they run into over charismatic han solos, they follow the true words of his lord darth vader. they also have the death star.
Too bad that can't hit anything ~_~
Well skill depends on the warrior so the best way to argue this is age and technology
Vikings used iron weapons, wooden shields, mail or no armor
Spartans used bronze and fought in formations so 1v1 Spartan is not that special
Knights ranged over a wide time period but posses the greatest technology and steel quality
Samurai had poor quality steel in the form of sand steel known as tamahagane but developed great simihing techniques. They wore leather armor and focused on spear, archery, horse riding, and lastly sword fighting
Gladiators were mainly slaves or captured fighters and would not excel against a professionaly trained soldier
Romans fought with bronze weapons as well but technology and formations > Spartans
1v1:
Knight > Viking > Samurai > Roman > Spartan
in groups
Knight > Viking > Roman > Spartan > Samurai
In the end this will depend on the commander and the skill of the warriors
EDIT: sorry thought amazons were spartans. My bad
I find your parameters incorrect. Technology is a part of it, but you based your answer way too much on it and you left other more important stuff out of it. Also you should judge them based on their own age and not in comparison with other ages as it really doesn't matter if you use bronze or iron when your opponent is using the exact same thing. Hence focusing so much on their equipment as you do, is not very useful to make a more correct assessment. By default it's also pointless to talk about 1 vs 1 as that first and foremost depends solely on the skill of the warrior. A man without armor or weapons could beat a fully armored knight if he knows how to fight.