The antonym of "un-fussiest" is "fussiest," which means someone who is very particular, hard to please, and demanding. The fussiest person always wants things their way, and nothing seems to satisfy them. They are often nitpicky and overly concerned with details, which can be exhausting for others. The opposite of being un-fussy is being fussy, which implies that someone is high maintenance and requires a great deal of attention to keep them happy. So, if you are looking for a low-maintenance person who is easy-going and laid back, you should avoid someone who is fussy and, instead, look for someone who is un-fussy.