“Best.” It means “Best wishes, but I can't be bothered to type six letters, so not really best wishes.” It is like ending your email with a splat of Vaseline or Marmite. Or a googly eyed face that turns out to be on a middle finger raised in insult.
As I learned at school and still practice, there is a sequence of signoffs. It goes:
Yours faithfully [which you keep if you are talking to Gandhi or other luminary who is truly cool]
With best wishes
“Best.” It's a sonic and social splat. No wonder we all hate one another.