Yes, COMMONEST is a Scrabble word!

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

Is it a valid word?

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

belonging to or participated in by a community as a whole; public
having no special distinction or quality; widely known or commonly encountered; average or ordinary or usual
common to or shared by two or more parties
commonly encountered
being or characteristic of or appropriate to everyday language
of or associated with the great masses of people
of low or inferior quality or value
lacking refinement or cultivation or taste
to be expected; standard

Also: rough-cut.