1,. "I remain" is completely archaic. It is the epistolary equivalent of "Egregissimo" in Italian.
The full, formal 19th century version was "I remain, sir, your obedient servant... John Smith"
2. In school (40 years ago) we were taught that Sincerely (or Yours sincerely) is used ONLY in personal correspondence, Yours truly ONLY in commercial/ business correspondence.
3. With email, everything has become more liberal. As email is a less formal medium, it seems paradoxical to re-introduce very formal expressions (I remain; Yours truly)
4. "Best" could be an abbreviation of best regards or the more personal Best wishes.
My brother occasionally uses "Best" in his emails to me. I do not like it. It sounds "breezy" and insincere. It also suggests that the person cannot make the effort to write a complete phrase!
Among my men friends, "All the best" is popular -- a friendly but not too affectionate farewell.
Happysummer: I was taught that the second word is never capitalized: eg Many thanks, not Many Thanks.