Its a very difficult choice because its not that specific, if i do i a little bit of research even on the ones i do not know very well i can find many honerable things about them, but it depends very much on the time periods we are looking at.
Before the age of ghengis khan, the mongels had a very unorganised society consisting of scattered tribes, bandits, merchants, common folk and thieves- they were only really interested in their own affairs back then. When ghengis khan came along with his bloody conquests and united the mongels and the vast bulk of the empires and nations in the world at that time, the mongels really came into their own as far as warriors and a new modern society is concerned.
I certainly wouldn't call the roman gladiaters the best warriors of all time though, any up-to-date historian will tell you now days that most gladiators were just common criminals or poor people dragged off the streets and pitted in the stadiums together to fight it out for their lives, or get slaughtered in a huge variety of barbaric and cruel ways for entertainment often with no hope of survival.
European knights, well, thats really vague...If you are talking about the english knights of chivalry and morals, well, some knights were very honerable, especially those close to the queen, but most were just thugs who did what they pleased while not called into service and went around murdering each other for their weapons or posessions and similar things.
Most people view the vikings as migrants who moved to the shores of britain for trade, wealth and more importantly- farming. Yes, most of the vikings were simply tribal lords who heard of the fertile varied lands we have over here, in comparsion to their sparse unfertile soils, and came over here for a better life in general. Its true that some vikings did the whole traditional "lets rampage through the monastarys and kill all the monks, burn the books and steal the relics, tapestries and other priceless items", but this was mostly in the name of funding their oversea's campaigns, but most vikings were simply interested in the farming prospects over here.