dullest

lacking in liveliness or animation;
emitting or reflecting very little light;
being or made softer or less loud or clear;
so lacking in interest as to cause mental weariness;
(of color) very low in saturation; highly diluted;
not keenly felt;
slow to learn or understand; lacking intellectual acuity;
(of business) not active or brisk;
not having a sharp edge or point;
blunted in responsiveness or sensibility; - Willa Cather
not clear and resonant; sounding as if striking with or against something relatively soft;
darkened with overcast;

dullest is a valid Scrabble (US) TWL word

dullest is a valid Scrabble Word in Merriam-Webster MW Dictionary

dullest is a valid Scrabble Word in International Collins CSW Dictionary

dullest is a valid Words With Friends word