Yes, FIERCEST is a Scrabble word!

Letter values in Scrabble US: F 4, I 1, E 1, R 1, C 3, E 1, S 1, T 1.

Is it a valid word?

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

marked by extreme and violent energy
marked by extreme intensity of emotions or convictions; inclined to react violently; fervid
ruthless in competition
violently agitated and turbulent

Also: trigger-happy.