Swedish, Norwegian, Danish. Which language do you think is the most useful and why?

Swedish is like International language of Nordic Countries, so speaking Swedish can help you to be understooble in Norway and Denmark. but also in Finland. Swedish also more widely spooken. But if you have strong accent or speak Swedish bad, people can just swithch to English especially in Sweden. Norwegian is also a good choice since Norwegians mostly respect people who learn their language, and it also can help you to understood some writen Danish, because Norwegian Bokmål is based on Danish language. Language of Denmark have the smallest native speakers number of that language you noticed. It is also more harder than Swedish and Norwegian, because the way how words writed and pronounced is a bit diffirent. Danes not pronounce most of the letters, like they pronounced Amager as "A'ama", and Andersen as "A'nsen". Or try to pronounce they famous Røde grøde med fløde. They also mostly respond in English if you have an accent, and it is very hard to speak that language. Even many Danes not understand each others. So my answer is Swedish.

Learn Norwegian -- it sounds kind of like Swedish, but looks like Danish. With good will you can use Norwegian in both Denmark and Sweden. I was at a Nordic lexicography meeting with Swedes, Norwegian, and Danes where everyone spoke his/ her mother tongue (Finns spoke Swedish; Icelanders spoke Danish).

More people speak Swedish than Norwegian, Danish or Icelandic. However, the languages are broadly similar and if you learn one, you'll have little difficulty learning the others.

Danish in Copenhagen

Swedish in Stockholm

Norwegian in Oslo.

out of the three, Swedish. however, there are many many languages outside of those three which are one hundred times more powerful e.g. Spanish and German.

None of the above. English, then Spanish, then perhaps French or German. Chinese or Japanese would be high on the list as well. I'd even put Portuguese above the ones you listed.

Sweden its a lot business there.