I know they were successful raiders, traders and very good sailors. BUT- They didn't win all battles, far from it actually, but they did when invaded some important places. But what's so great about conquering England? Different from what media makes us think, Medieval England was not advanced, at the time some few castles doesn't mean that. Most of the Middle East was more advanced than Medieval England, they were remains, fragmented kingdoms of a fragmented Empire, and with economic issues. I know they conquered Russia but not even 10% of the current territory, and before Kiev Rus it was a brunch of slavic tribes (well, I don't know if all of them were like that, but most of them), and we all know slavs suffered a lot and even were captured by West Asians in Arab slave trade. And before we talk about Sicily and other palaces like Constantinople, we have to know that normans weren't vikings, ethnically speaking. They were a people that descend of vikings and married locals and it happen to call them vikings because of marketing maybe, but they were as french, in two generations, even more french than the past generations. They called themselves french, spoke their own french, adopted french culture and were christians. I am talking about the Vikings themselves, the pure viking culture, not christian with norse blood that adopted the culture of the Roman Empire. But I don't know all. There must be something that makes them greatest warriors of their time (not of Europe because, speaking of early medieval times, Europe wasn't really a great thing, far from it, they were even "ugh").
Source: reddit post