Yes, lightest is a Scrabble word!

Adjective

1. Of comparatively little physical weight or density
2. (used of color) having a relatively small amount of coloring agent
3. Of the military or industry; using (or being) relatively small or light arms or equipment
4. Not great in degree or quantity or number
5. Psychologically light; especially free from sadness or troubles
6. Characterized by or emitting light
7. (used of vowels or syllables) pronounced with little or no stress
8. Easily assimilated in the alimentary canal; not rich or heavily seasoned
9. (used of soil) loose and large-grained in consistency
10. (of sound or color) free from anything that dulls or dims
11. Moving easily and quickly; nimble
12. Demanding little effort; not burdensome
13. Of little intensity or power or force
14. (physics, chemistry) not having atomic weight greater than average
15. Weak and likely to lose consciousness
16. Very thin and insubstantial
17. Marked by temperance in indulgence
18. Less than the correct or legal or full amount often deliberately so
19. Having little importance
20. Intended primarily as entertainment; not serious or profound
21. Silly or trivial
22. Designed for ease of movement or to carry little weight
23. Having relatively few calories
24. (of sleep) easily disturbed
25. Casual and unrestrained in sexual behavior