Yes, FALSEST is a Scrabble word!

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

Is it a valid word?

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

not in accordance with the fact or reality or actuality
arising from error
erroneous and usually accidental
deliberately deceptive
inappropriate to reality or facts
not genuine or real; being an imitation of the genuine article; it's real synthetic fur"
designed to deceive
inaccurate in pitch
adopted in order to deceive
(used especially of persons) not dependable in devotion or affection; unfaithful

Also: off-key, put on.