The phrase "most nonextant" essentially means "virtually nonexistent." Therefore, antonyms might include words that denote abundance, prevalence, or existence. Such words could include "plentiful," "abundant," "ample," "prevailing," "widespread," or "existing." Other options might include phrases such as "highly prevalent," "abundantly present," or "widely distributed." Depending on the context, other antonyms for "most nonextant" could include "common," "well-established," "frequently encountered," or "easily accessible." Ultimately, the specific antonym chosen will depend on the context and intended meaning of the sentence.