The term "whacko" is a slang term used to describe someone who is considered to be crazy or mentally unstable. Antonyms for "whacko" would be terms that describe someone who is rational, sane, and mentally stable. Such antonyms could include terms like "normal," "sensible," "level-headed," "balanced," "well-adjusted," "reasonable," and "sane." These antonyms can serve as a reminder that not everyone who behaves in a way that seems strange or unusual is necessarily mentally ill, and that it is important to treat all types of individuals with respect and empathy, regardless of their behavior or appearance.