The liquid with the lightest density is hydrogen gas, at a density of 0.08988 g/L at standard temperature and pressure (STP).

It's important to note that hydrogen is a gas at room temperature. To get liquid hydrogen, it needs to be cooled to extremely low temperatures (-252.87 °C or -423.17 °F).

Here are a few other liquids with relatively low densities:

* Helium (liquid): 0.125 g/L at its boiling point (-268.93 °C or -452.07 °F)

* Neon (liquid): 1.204 g/L at its boiling point (-246.08 °C or -410.94 °F)

* Nitrogen (liquid): 0.808 g/L at its boiling point (-195.79 °C or -320.42 °F)

* Oxygen (liquid): 1.141 g/L at its boiling point (-182.95 °C or -297.31 °F)

These are all very light liquids, but hydrogen gas is the absolute lightest.