Answer: blithest

AnswerCrossword Clue
BLITHESTMost joyful
BLITHESTMost carefree
BLITHESTMerriest
BLITHESTMost merry
blithestBLITHE, merry (mirthful also MERRIE)
blithestlighthearted; carefree