Is there an "easiest" Nordic language?
Of all the Nordic languages on here is one of them easier than the other? They all look extremely similar to me.
None of them are terribly hard for an English-speaker, but Swedish is slightly grammatically simpler than Norwegian (albeit a little less phonetic in writing), and both of these have much easier pronunciation than Danish (which is written considerably less phonetically).
Icelandic has a much more complex grammar to any of these three (and I think Faroese is to Icelandic rather as Afrikaans is to Dutch), but, as it isn't on Duolingo, I am not sure whether you are including it in your consideration.
There's not very much between Swedish and Norwegian (they are mutually intelligible to a very high degree). The Norwegian course on DL is longer, but the Swedish one is still very good. If you don't have a practical or academic reason for learning one or the other, I suggest you have a listen to people speaking both and see which one attracts you more.
all seem very simple compared to say Russian or Czech.
I'm finding Czech more difficult than Polish, and that's saying something!
Swedish, Norwegian and Danish are more like dialects than languages, linguistically. Norwegian have a simplified spelling since its written language is the youngest of these three. Both Icelandic and Faroese are very archaic compared to the Scandinavian languages. Finnish is not even remotely close to the other Nordic languages except Sami.