Strong: This is the most direct and common antonym.
Sturdy: Suggests something built to last and resist wear and tear.
Robust: Implies a strong and healthy build.
Solid: Describes something firm and not easily broken.
Durable: Something that is long-lasting and resistant to damage.
Substantial: Emphasizes a sense of weight and importance.
Rigid: Suggests unyielding and inflexible.
Tough: Implies resistance to pressure or force.
The best antonym will depend on the specific context in which you are using the word.