Danish. Swedish and Norwegian are just failed attempts at speaking Danish, proving once and for all that Norwegians have better language skills since their attempts are closer.*

Finnish is mainly good for swearing away bears and complaining about a lack of saunas.

*(Jk, in case there's any doubt)

Best for what purpose? Norwegian is best for understanding Swedish and Danish (Swedes have more difficulty understanding Danish than Norwegians do) Swedish and Norwegian are essentially a dialectical continuum.

Oslo dialect is heavily influenced by danish because back in the day if Norwegians wanted a decent education they went to Denmark. The more commonly used written form of Norwegian (bokmål) is still very similar to Danish (only as it's written though, danish pronunciation is fucked up). The other form (nynorsk) was created from several rural dialects as part of a nationalism movement and more closely resembles Swedish (not in terms of spelling but pronunciation) it's not as close as bokmål is to Danish though.

Finnish is a completely different language family but is very similar to kven which is spoken by very few people on the Northern coast of Norway. It's a nice sounding language though and a good way to sound like you're going to stab someone.

The correct language is the one that is spoken in the country you're most interested in. I live in Norway so for my purposes Norwegian is obviously best. And I don't need to revert to speaking English in Sweden or (most of) Denmark, which is nice.

If I had to learn a second north European language, I'd choose either northern Sami, Faroese, Icelandic, or Greenlandic.

Finnish, though I may be biased. We have the best curses (English swear words are soo boring and really don't have the same power.) and it's so easy to just make up bew words.

Danish is Swedish without opening your mouth and if it's half open, you get Norwegian

Finnish, hands down

Clearly Icelandic is best.