Best regards

You've seen both "Best regards" and "Best Regards" versions, haven't you?



Formally, "Best regards" is more suitable because formal letter rules mean that only the first word is a capitalized form. "Best Regards" is more common to appear in emails where formal rules are relaxed.
"Best Regards" is much less formal because it doesn't stick to the original letter rules that we are supposed to use. You're better off avoiding this (even in emails) so nobody calls you out for misusing your formal writing rules.