Yes, PROUDEST is a Scrabble word!

Letter values in Scrabble US: P 3, R 1, O 1, U 1, D 2, E 1, S 1, T 1.

Is it a valid word?

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

feeling self-respect or pleasure in something by which you measure your self-worth; or being a reason for pride
having or displaying great dignity or nobility