I often end my emails just with "Regards, FirstName".
But I also often see "Best regards", "Kind regards" and "Sincerely".
What is the difference in tone and meaning? Is one more formal than the other? Is there a difference between US and UK?
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I'm from the UK and I personally use 'Regards' on its own, like you. Even to somebody I don't know. That's usually for the initial contact type of email, when I'm first raising a subject with somebody. Subsequent responses tend to become less formal, with either a simple 'Thanks' or no ending at all.
However, from your list above, I most often see 'Kind Regards' and in fact I received an email with this today. I'd suggest you could rank those in order of formality: Sincerely > Best Regards > Kind Regards.
I've pretty much never seen 'Sincerely' in an email though, as I think the medium is intrinsically less formal than other methods. In my opinion, there are ways to increase the formality slightly, such as by avoiding contractions for example.