Yes, COOLEST is a Scrabble word!

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

Is it a valid word?

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

neither warm nor very cold; giving relief from heat
marked by calm self-control (especially in trying circumstances); unemotional
(color) inducing the impression of coolness; used especially of greens and blues and violets
psychologically cool and unenthusiastic; unfriendly or unresponsive or showing dislike
(used of a number or sum) without exaggeration or qualification
fashionable and attractive at the time; often skilled or socially adept