Antonyms for the word "most illusory" would suggest a sense of reality or authenticity. Some words that come to mind include "genuine," "true," "real," "accurate," "authentic," "factual," "verifiable," and "undeniable." These antonyms emphasize the importance of truth and factuality, which are essential for making sound judgments and decisions. By paying attention to these antonyms, we can avoid being deceived by illusions and misconceptions, and make informed choices based on reliable evidence and information. Ultimately, these antonyms offer a counterpoint to the fleeting and ephemeral nature of illusions, providing a firmer foundation for our understanding of the world around us.