Yes, coolest is a Scrabble word!

Adjective

1. Neither warm nor very cold; giving relief from heat

2. Marked by calm self-control (especially in trying circumstances); unemotional

3. (color) inducing the impression of coolness; used especially of greens and blues and violets

4. Psychologically cool and unenthusiastic; unfriendly or unresponsive or showing dislike

5. (used of a number or sum) without exaggeration or qualification

6. Fashionable and attractive at the time; often skilled or socially adept