Yes, LEFTEST is a Scrabble word!

Letter values in Scrabble US: L 1, E 1, F 4, T 1, E 1, S 1, T 1.

Is it a valid word?

LEFTEST is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

being or located on or directed toward the side of the body to the west when facing north
not used up
intended for the left hand
of or belonging to the political or intellectual left

Also: left over, left-hand.