I've long thought using "best" as your email close was insincere at best, and dismissive at worst. But how do other email closers come off? Bobulate's chart from 2007 tries to classify.
According to them, "thanks" captures the sweet spot between familiar and unfamiliar, natural and self-conscious. It's also the one I tend to use the most often when I'm dealing with people I've corresponded with only a few times. I'd actually argue that "best", "cheers" and "best regards" don't feel natural to me, but rather a little silted — but things might have changed between 2007 and now.
I'm also not sure why "sincerely" is in two almost opposite quadrants, but Essential Manners For Men author Peter Post claimed it was an all-purpose (and safe) way to sign off back in 2009. Though, it's so generic and cliched now people's eyes probably gloss over it without actually internalising the meaning of the word.
I'm curious what you guys use for your email closers. In the instances when I don't use "thanks", I tend to just leave it blank and not make the recipient spend another 100 milliseconds reading a superfluous word.