The antonyms for the word "unfussiest" are "picky," "fussy," and "particular." Someone who is picky is very selective and choosy, often making demands or expecting things to be done in a particular way. A fussy person is excessively concerned with small details, often to the point of being nervous or anxious. Someone who is particular is very precise or careful about what they want or how things should be done, often insisting on certain standards or requirements. These antonyms suggest a high degree of attention to detail and rigidity, which stands in contrast to the relaxed and easy-going nature of someone who is "unfussy" or "unfussiest.