The easiest of the Nordic languages for native English speakers is Norwegian. Its grammar and spelling is the most simple and regular. But more info about all of them under the cut.
Danish's pronunciation lacks the up-down tone/melody system that Norwegian and Swedish has to distinguish between words; as a replacement, it doesn't pronounce certain letters. It doesn't match its written (think of French or English) as much as the other ones do, though there is a pattern to it.
Norwegian and Danish have vastly simplified grammar compared to Swedish — all their plurals are the same, for example, but Swedish keeps the archaic, various plural forms based on back when there were three genders (=loosely based on old Norse), and has some archaic phrasing that the other two can't do.
— "Bokmål" (literally: book language) or "west Norwegian" is the version of Norwegian based off Danish. They were forced to read and write in Danish for a long time so that's why it's called that. It's the one with simplified grammar.
— "Nynorsk" ("new Norse"; new Norwegian) is the form of Norwegian that's just like Swedish in grammar. To someone who knows Swedish, this just sounds like a Swedish dialect.
— Norwegian is like a bridge between Danish and Swedish in pronunciation and spelling; plus it's easiest to learn. It's also closer in vocabulary to Faroese, Icelandic and Old Norse compared to the other two.
— Swedish is the biggest Scandinavian one; if you want fun practise materials, it's easier in Swedish.
— The "Finland-Swedish" varient of Swedish is like Danish; it lacks the tonal up-down system and uses slightly different grammar/phrasing and vocabulary. However, overall it's more like Sweden-Swedish than Danish is.
Learning one Scandinavian language, it's a simple jump to understanding the other two. It takes very little effort and as far as the written languages are concerned, you can usually read them without even looking words up. This is because they're not actually separate languages, and for the longest time they weren't considered separate either, but they're just dialects of each other (that are just called languages nowadays for political reasons). If your Swedish gets to the point where you can understand Northern, Southern, or Finland Swedish too then you'll also understand Norwegian and Finland-Swedish without a problem. Danish comes with a bit more practise, especially as Danish is usually "translated" into Swedish and isn't broadcasted so much in Sweden compared to Norwegian.
The mother of all Nordic languages except Sami, Finnish and Greenlandic. It's the language of the vikings and is extremely closely related to ancient English (called "Anglo-Saxon"). There's extremely little difference between it and modern Icelandic - the main difference between the two is that Old Norse has less words overall, some of its grammar is more regular, and all you have as reading practise in it is ancient texts. Icelanders all study old Norse in school and if you get a degree in Icelandic in Iceland you're required to study it as well, in your second year (at least, it was so in 2011).
Learning Danish or Norwegian first will jump-start your Icelandic, Faroese or Old Norse learning. (If this is your goal, don't learn Swedish as the more difficult grammar is a total waste of time). All the grammar in those languages are taken entirely for granted when you learn one of the other three — all the basic vocabulary is the same between the languages, and textbooks for Icelandic might not even mention the grammar that's the same between them too (as they always focus on the more difficult grammar). I'm speaking from experience, teachers in Iceland expect or hope you already know a Scandinavian language.
4-5 times harder than any Scandinavian language to learn, and maybe 2 times harder than Faroese. There are many more aspects to the grammar, less materials to learn from (it's harder to find cartoons dubbed in Icelandic, translated comics, textbooks for it, etc). Between it and other languages (including Scandinavian), there aren't so many words in common.
— Icelanders are not very open to learners, and the language is very irregular in things like plural forms getting sound changes. The Icelandic-English dictionaries are AWFUL.
— An English-native I met became fluent in it after five years when living outside of Iceland. A German-native I met was almost fluent after two years.
— The language has overall changed so little that even if your learning materials are 200+ years old it actually doesn't matter (you'll only be missing some modern vocabulary). If you've studied old Norse, in general the same is true.
In writing: a mix of 70% Icelandic, 20% Scandinavian and 10% itself.
In speaking: 50% Icelandic, 20% Scandinavian 30% itself. It's kinda like as if Icelandic and Dutch were mixed.
It's a simplified, regluarized version of Icelandic. Basically, all the difficult grammar and vocabulary in Icelandic was replaced with easier stuff from Scandinavian. The spelling doesn't match the written for the same reason as Danish; if they changed it, it'd be a lot harder for other Nordic people to learn their language and vice versa.
If you know Icelandic and a Scandinavian language already you can simply pick up a book and read it without ever needing lessons or dictionaries. Partially for that reason, there's basically zero textbooks for it. Shipping for any sort of practise materials, like novels, are incredibly expensive in most cases — luckily enough there's tons of writing and videos online you can get for free.
— A Faroe Islander can become fluent in Icelandic after living in Iceland for just two months.
— Faroers are much more proficient in Scandinavian than Icelanders are, and they also understand (for the most part) Icelandic. Young Icelanders on the other hand, have no clue about Faroese and can't really understand it spoken (older Icelanders can). The English level between the two countries is seemingly about the same, but as I've never lived in the Faroes I don't know for sure.
Completely unrelated to the previously-mentioned languages. However, tons of bilingual materials and learning materials can only be found in Swedish for it. If you know Swedish, your job opportunities in Finland are better and you can get citizenship faster.
Finns are, like Icelanders, not very open to learners, but the language is very regular and is widely found online.
— I met an English native speaker who became fluent in Finnish in four or five years, but part of that time was spent actually living in Finland.
— Lessons teaching Finnish are known to be awful. Just like Icelandic, they're normally intended for people who actually live with native speakers, or who already "know" how to speak the language as ex. their parents speak it.
SAMI / LAPPISH
The collective name for the various indigenous languages of the once-nomadic people who lived in the north of Finland, Russia, Sweden, and Norway. They can be as different from each other as English and German, that is to say, a speaker of one doesn't necessarily understand a speaker of another.
— All, and I do mean all, of the learning resources are in Scandinavian, Finnish or Russian. Even the ones that exist are old and very shitty. Even if you live in Sweden you can't learn Sami just anywhere, you have to actually live in the North of Sweden to find courses.
— A lot of learning materials and classes are restricted towards people who are Sami themselves or who are married to Sami people. Yeah. This isn't something Scandinavia enforces, it's what the Sami people themselves enforce.
From what I've seen, the grammar is similar to Finnish grammar and the textbooks will assume you've already learnt Finnish.
From all that I've heard, Sami speakers are not very open to learners, especially if you're not Sami or of another indigenous culture yourself but including even to other Sami people. You won't be able to find hardly any practise material in it, as they don't ex. translate popular books (Harry Potter etc) to it.
Completely different from the others yet again. It's a dialect of the "Inuit" or "Aleut" languages, meaning that the entire western half of Canada speaks the same language as Greenland, with extremely little differences between the two. There are almost no learning materials in English for Greenlandic; neither lessons nor dictionaries. You'll have to learn Scandinavian (=Danish) or German. The Greenlanders themselves aren't very good at either Danish or English unless you're in the capital or a "Danish town", or they've actually lived abroad.
In the main Nordics, words put together average on less than 3 words at once. In Finnish, Sami and Greenlandic there are more at once (called "cases" but they're actually separate words that just fused at the root).
In Greenlandic, there's no distinction between any part of speech (adjectives, adverbs, verbs, nouns, tense) and to make the meaning clear, more words are suffixed on to the "main idea". In the fact that there's no real parts of speech and it's just word order or suffixes that clarify things, Indonesian, Chinook Jargon and Chinese are exactly the same way. In Greenlandic's case it looks like this:
dog. dog-red. dog-red-now. dog-red-is owned-now. I-own dog-red-is owned now.
= i have a red dog.
Greenlandic is actually extremely simple and the language is extremely regular (even moreso than Finnish, from what little I know), but the dictionaries are so bad that they don't list individual words and only list compound words that are really like entire sentences!!
Relatively recently (in around the 70's) there was a large spelling reform, so if you learn from older materials you won't be able to understand modern ones as easily. There are convertion tools though.