There are many antonyms for the phrase "cracked brains," which commonly refers to someone who is foolish or crazy. Some possible antonyms include intelligent, rational, sensible, astute, insightful, and level-headed. These words all suggest someone who is capable of making sound decisions and thinking critically, rather than being prone to wild or erratic behavior. Additionally, terms like "sane," "composed," and "together" suggest that the person is in control of their thoughts and actions, rather than being overwhelmed by emotions or delusions. Ultimately, the antonyms for "cracked brains" speak to the importance of clear thinking and common sense in our lives.