Yes, STRANGEST is a Scrabble word!

Letter values in Scrabble US: S 1, T 1, R 1, A 1, N 1, G 2, E 1, S 1, T 1.

Is it a valid word?

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

being definitely out of the ordinary and unexpected; slightly odd or even a bit weird
not known before
relating to or originating in or characteristic of another place or part of the world