Several substances expand on freezing. Gallium expands 3.1% and bismuth 3.3%. However, both of these are much less than water which expands 9%. Is there any substance that expands more than 9% on freezing?
We can generalize this question to any system at thermodynamic equilibrium and constant pressure. Density normally decreases with temperature, but sometimes increases. For example, heating plutonium increases it's density in three places: a solid-solid phase transition, a negative thermal expansion, and melting. These effects combine to make the density increase about 5% between ~320C to 640C, which is still less than cooling 4C water to -0.01C ice.
If we allow substances that dissolve/precipitate and reversible chemical reactions can we beat water? We do not allow super-cooling or super-saturation since that isn't thermodynamic equilibrium.