Learn English – When did the valediction ‘best regards’ come into use

When working in England, I've found that "Regards" is most common, even amongst parties that have a good relationship.

When working in Australia, "Best regards" is much more commonly used. Using simply "Regards" would seem quite standoffish.

I'm not sure which is most applicable in Denmark.

In formal writing you would just continually use "dear." See for example the correspondence between Vladimir Nabokov and Edmund Wilson, titled appropriately, Dear Bunny, Dear Volodya. In many (North American) business contexts, subsequent letters omit the greeting in favor of a more informal, chat-like style. However, "dear" is pretty neutral and almost always appropriate.

There are no settled greetings in informal correspondence. Much depends on the context and on the status of the person you are addressing (to state the obvious). Most importantly, the greetings and the sign-offs add a much-needed touch of personal flair. You can start a "hey" or a "greetings" or whatever else works for you. Pick your approach, and use it consistently.

As long as your tone is appropriate to the situation, you cannot go too wrong.