The substance that has the lowest freezing point at normal atmospheric pressure (1 atm) is actually helium-3 (³He). However, since you mentioned not to include helium, the substance with the next lowest freezing point is hydrogen (H₂).
Hydrogen has a boiling point of -252.87 degrees Celsius (-423.17 degrees Fahrenheit) and a melting point of -259.16 degrees Celsius (-434.49 degrees Fahrenheit) at normal atmospheric pressure. At such low temperatures, hydrogen exists as a liquid and exhibits remarkable properties, including extremely low viscosity and high thermal conductivity.
It's worth noting that there may be other exotic substances or mixtures with even lower freezing points under specific conditions, such as ultra-high pressures or modified atmospheres. However, under normal atmospheric pressure, hydrogen has the lowest freezing point among commonly known substances.