Yes, MOSTEST is a Scrabble word!

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

Is it a valid word?

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

(superlative of `many' used with count nouns and often preceded by `the') quantifier meaning the greatest in number
the superlative of `much' that can be used with mass nouns and is usually preceded by `the'; a quantifier meaning the greatest in amount or extent or degree