Answer: lightest

AnswerCrossword Clue
LIGHTESTLeast burdensome
LIGHTESTLeast serious.
LIGHTESTWhat hydrogen is.
LIGHTESTMost buoyant
LIGHTESTLeast heavy
lightestLIGHT, having little weight
lightestHaving a considerable or sufficient amount of natural light; not dark
lightest(of a color) Pale