Yes, TOUGHEST is a Scrabble word!

Letter values in Scrabble US: T 1, O 1, U 1, G 2, H 4, E 1, S 1, T 1.

Is it a valid word?

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

not given to gentleness or sentimentality
very difficult; severely testing stamina or resolution
physically toughened
substantially made or constructed
violent and lawless
feeling physical discomfort or pain (`tough' is occasionally used colloquially for `bad')
resistant to cutting or chewing
unfortunate or hard to bear
making great mental demands; hard to comprehend or solve or believe