Yes, best is playable
...and is worth 7 points.
Found in the following dictionaries
Enable (Words with friends)
SOWPODS (Collins Scrabble Words)
OTCWL2014 (NASPA Word List 2014)
TWL06 (NASPA List 2006)
"Best" Definition
noun
One that surpasses all others.
The optimum condition or quality: look your best. She was at her best in the freestyle competition.
The supreme effort one can make: doing our best.
idiom
At best Interpreted most favorably; at the most: no more than 40 people at best in attendance.
At best Under the most favorable conditions: has a top speed of 20 miles per hour at best.
For the best With an ultimately positive or preferable result.
Get To outdo or outwit; defeat: My opponent got the best of me in the debate.
Make the best of Accept (a bad situation) in as good a light as possible.
adjective
Greatest; most: He spoke for the best part of an hour.
Most highly skilled: the best doctor in town.
Most satisfactory, suitable, or useful; most desirable: the best solution; the best time for planting.
Surpassing all others in excellence, achievement, or quality; most excellent: the best performer; the best grade of ore.
adverb
In a most excellent way; most creditably or advantageously.
To the greatest degree or extent; most: "He was certainly the best hated man in the ship” ( W. Somerset Maugham).
In the best way to greatest advantage.
To the highest degree; most (used with verbs suggesting a desirable action or state or a successful outcome):.
verb
Outwit or get the better of (someone):.
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