I'm mostly with gettingfat, but real life is complciated .....
The Samurai trained as specialists in hand-to-hand combat. But in practice, horse archery was powerful and in battles they fought mostly on horseback with bows; of course, they could fight in hand-to-hand combat and in ceremonial combats.
For the most part, the Elite combat troops of Western Europe were knights, trained to fight not with bows but with shock on horseback. Europe, of course, did have hand-to-hand troops. They had Seargents at-arms, which were well trained, but not with the training to match the Samurai in general.
The best footsoldiers of the Medieval period were the Swiss pikemen, who were often viewed as invincible. Pikemen fought IN FORMATION -- take a pikeman alone, his lack of ability to move laterally or defend behind will be telling, but in formation, they were a terror.
So, the question, without context, is silly:
A samurai, in an arena, was well trained, probably better than a Sergeant at arms
Using their weapons, a Samurai's sword was far superior to anything in the west. However, a dismoutned knight also learned to fight in hand-to-hand combat, but probably 2/3 of his training was fighting on horseback. conversely, his heavy armor may protect against the cutting surface of the samurai sword. So, a dismounted knight with REALLY GOOD armor like plate-mail will be tough to take. Even with chainmail, a samurai sword may not the right weapon to defeat it, but the samurai would be tough then.
On horseback, knights vs. Samurai is simply archers against shock. The samurai could fight with shock, but they didn't train at it like the Western knights.
Against pikemen, if the samurai were on foot, their training wouldn't help. In formation,the pikemen were very strong. If the samurai fought the pikemen one-on-one, the samurai would slaighter them.
Remember the samurai against the Mongols. The Mongols were doomed because they were caught on the beach and couldn't sue their mobility, but they still caused disproporationate losses because they fought in formation.
Best wishes,
Breunor