The word "plainliest" is a superlative form of "plain," meaning the most simple, unadorned, or uncomplicated. Antonyms for "plainliest" would include words that suggest complexity, ornamentation, or elaboration. Some examples of antonyms might include "fancy," "elaborate," "ornate," "intricate," "embellished," "complex," and "sophisticated." These words all suggest a level of refinement or detail that is absent from the concept of plainness. Other antonym options might include "baroque," "lavish," "ornamented," or "exquisite," all of which evoke a sense of detail and beauty beyond what could be described as "plain.