Grimmest rhymes with:
grimaced, grimaced, grimaced, slimmest, alarmist, anatomist, animist, biochemist, calmest, dumbest, firmest, gamest, grimaced, pessimist, premised, promised, slimmest, taxidermist, warmest
Grimmest sounds like:
garnished, germicide, granquist, greenest, grimaced, grimstad, grinstead, gronseth