Honesty: This is the most straightforward opposite. It means being truthful and straightforward in your words and actions.
Truthfulness: This is similar to honesty but emphasizes the act of telling the truth.
Integrity: This encompasses a broader sense of ethical conduct, including honesty, reliability, and consistency between words and actions.
Transparency: This implies openness and clarity in communication, making sure that all relevant information is accessible.
Candor: This refers to being frank and outspoken, even if it might be uncomfortable.
Frankness: Similar to candor, this means being direct and honest in your communication.
Veracity: This is a formal term for truthfulness and accuracy.
The best opposite for deceit depends on the context and what aspect you want to highlight.