Which state of matter has the highest entropy?
Entropy by definition is the degree of randomness in a system.
If we look at the three states of matter: Solid, Liquid and Gas, we can see that the gas particles move freely and therefore, the degree of randomness is the highest.
For liquid state, still the particles are moving but less freely than the gas particles and more than the solid particles.
Here is a video that explains more this topic and how it is related to spontaneity.
Thermodynamics | Spontaneous Process & Entropy.