Viking. It's not much of a contest and the fact that the show got it so wrong is proof toward this. People think of Vikings as mindless berserkers, but they were incredibly smart opponents, usually short on numbers and so absolutely requiring their own survival through numerous combats. They did not have the best gear, but they had incredible tactics, a battle training that, while crude, was very effective, and the gear they did have matches up incredibly well against samurai.

For instance, the katana (and about 90% of samurai melee weapons, for that matter) was made as a slashing weapon against unarmed or lightly armored opponents. For this purpose it excelled, but the katana never came up against chain mail or, worse, plate. Chain mail, fairly basic armor for a viking, was made for the sole purpose to stop slashing weapons. Through testing today we know that the katana would not cut through this, meaning on a basic viking opponent they would have to ignore the entire torso and arms and on a particularly rich or successful viking there would only be a bare few joints in the helmet or legs they could attack. Add a shield into the mix and that's even more they have to get around. Or if they try to parry against one of the larger weapons a viking owns their own weapon is very likely to break. On the reverse side, most of the weapons a viking would use would incapacitate or severely damage with a glancing hit anywhere on the body, not to mention either break through the armor the samurai is wearing or just the bones underneath. In a melee fight a viking has every edge.

Not to say Samurai have nothing going for them. They had better bows and better expertise at a range. But Vikings were not really the types to go blindly charging in a wide open space. Again, they had to be smart about survival. They did not have great numbers. Even so, at a range I'd take a samurai. At anything else I'd take a viking. And as a complete warrior, definitely a viking. Better tactics, better gear. Get a good berserker shot up on painkillers with good armor and he'd bulldoze just about any samurai who got in his way.

Really, there is no honest argument for samurai unless you want to go purely ranged. And that would just make a landslide victory into a helluva battle.