Thermodynamic stability of benzene derivatives
Wikipedia defines thermal stability as . 


  Thermodynamic stability occurs when a system is in its lowest energy state, or chemical equilibrium with its environment. This may be a dynamic equilibrium, where individual atoms or molecules change form, but their overall number in a particular form is conserved.  


For the particular problem in hand the lowest energy state definition is more relevant for us .      

Enthalpy of formation can be used for judging the energy states.  

A  negative enthalpy change implies that the product has released heat thus lowering the amount of energy present in the product 

This implies. 


  More negative the enthalpy of formation, more stable the compound.  




Enthalpies of formation    


Phenol
$\ce{ -&gt; -165.0 kJ/mol}$.      
Hydroquinone
$\ce{-&gt; -371.1 ± 1.3 kJ/mol}$     
P-Nitrophenol
$\ce{-&gt; -207.1 ± 1.1 kJ/mol}$   


 
All data has been taken from http://webbook.nist.gov , for solid state. 
 



The order can be calculated now as         

Hydroquinone $&gt;$ paranitrophenol $&gt;$ phenol