Yes, BEST is a Scrabble word!

Letter values in Scrabble US: B 3, E 1, S 1, T 1.

Is it a valid word?

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

the supreme effort one can make
the person who is most outstanding or excellent; someone who tops all others
Canadian physiologist (born in the United States) who assisted F. G. Banting in research leading to the discovery of insulin (1899-1978)

(superlative of `good') having the most positive qualities
(comparative and superlative of `well') wiser or more advantageous and hence advisable
having desirable or positive qualities especially those suitable for a thing specified
having the normally expected amount
morally admirable
deserving of esteem and respect
promoting or enhancing well-being
agreeable or pleasing
of moral excellence
having or showing knowledge and skill and aptitude
thorough
with or in a close or intimate relationship
financially sound
most suitable or right for a particular purpose
resulting favorably
exerting force or influence
capable of pleasing
appealing to the mind
in excellent physical condition
tending to promote physical well-being; beneficial to health
not forged
not left to spoil
generally admired

in a most excellent way or manner
it would be sensible
from a position of superiority or authority
(often used as a combining form) in a good or proper or satisfactory manner or to a high standard (`good' is a nonstandard dialectal variant for `well')
thoroughly or completely; fully; often used as a combining form
indicating high probability; in all likelihood
(used for emphasis or as an intensifier) entirely or fully
to a suitable or appropriate extent or degree
favorably; with approval
to a great extent or degree
with great or especially intimate knowledge
with prudence or propriety
with skill or in a pleasing manner
in a manner affording benefit or advantage
in financial comfort
without unusual distress or resentment; with good humor

Also: c. h. best, charles herbert best, in effect, in force.