POSSIBLEST is a playable word

possible Scrabble® Dictionary

adjective
possibler, possiblest
capable of happening or proving true
(adverb) possibly
noun
pl. possibles
a possibility
373 Playable Words can be made from "POSSIBLEST"
   2-Letter Words (22 found)
   8-Letter Words (5 found)
   9-Letter Words (1 found)
   10-Letter Words (1 found)