"Brieriest" is a word that describes the thorniest and most dense of bushes or thickets. Its antonyms would therefore be words that describe the opposite of such a thorny and dense vegetation. Some antonyms that come to mind include words like smooth, soft, gentle, and easy. These words evoke images of grassy meadows, silken petals, and calm breezes, which are the complete opposite of a rough and prickly briar patch. Other antonyms for "brieriest" could include words such as sparse, thin, simple, and straightforward. These words suggest a lack of complexity and density, and therefore a lack of thorns and prickles.