Antonyms for the phrase "most plain-spoken" could be words like ambiguous, cryptic, evasive, mysterious, or guarded. When someone is not being plain-spoken, they might use language that is intentionally vague or unclear, leaving their true intentions or feelings open to interpretation. They might also withhold information or use misleading language. While being plain-spoken can sometimes come across as rude or blunt, it is often appreciated for its honesty and directness in communicating important ideas or feedback. However, it is important to consider the audience and situation when choosing how to communicate, as being too plain-spoken can sometimes be inappropriate or counterproductive.