m-Xylene is the most stable.
This has been explained on the basis that the m-isomer is the thermodynamically controlled product, whereas the other two are kinetically controlled products. These results are explained in terms of basicity. Rearrangement involving hydride shift ( instead of Me) can also occur ( during isomerisation reaction).Only for the m-carbonium ion the two methyl groups ( +I) in one of the canonical structures is capable of partially neutralising the positive charges on the ring carbon atoms. In the other two isomers only one methyl group can act in this way. Thus the m-carbonium ion will be more stable than the other two and so the equilibrium is shifted to the formation of m-isomer.