Yes, DEADEST is a Scrabble word!

Letter values in Scrabble US: D 2, E 1, A 1, D 2, E 1, S 1, T 1.

Is it a valid word?

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

no longer having or seeming to have or expecting to have life
not showing characteristics of life especially the capacity to sustain life; no longer exerting force or having energy or heat
very tired
unerringly accurate
physically inactive
(followed by `to') not showing human feeling or sensitivity; unresponsive
devoid of physical sensation; numb
lacking acoustic resonance
not yielding a return
not circulating or flowing
not surviving in active use
lacking resilience or bounce
out of use or operation because of a fault or breakdown
no longer having force or relevance
complete
drained of electric charge; discharged
devoid of activity; nothing ever happens here"

Also: all in.