Hazardous

The most appropriate antonym of the word "hazardous" is safe.

• Hazardous means "likely to cause harm or danger."
• Safe means "not dangerous or risky."

The other words do not have the same meaning as "hazardous" and are therefore not antonyms.

• Unassuming means "not claiming importance or significance."
• Perilous means "full of or involving danger."
• Freak means "an unusual or unexpected event or occurrence."