By the numbers, Swedish looks like the most promising. Swedish 9million, Danish spoken by 5million, Norwegian 5million, Icelandic 300,000. Finnish is a different language family from the first four, so if one is considering Finnish, might as well consider other agglutinative languages, but that is another post.
Norwegian is two official dialects (I think they are mutually intelligible, maybe even dialects), I’ve heard that Swedish and Norwegian are almost mutually intelligible. As far as I know, no other pairs of the above languages are mutually intelligible.
Danish is spoken on some other islands far away, like Greenland and the Faroe Islands.
I still like Icelandic better, because, hey, they’re closer than Norway, Sweden or Denmark.
Any how. Today I foud some Icelandic podcasts! http://www.mbl.is/mm/podcast/
Now I can hear a critical mass of Icleandic vowels and consonants so I can start to reproduce them instead of making wild guess from how the word looks on the page.