Hitmonchan is better than Hitmonlee. Hitmonchan possesses superior physical and special statistics and more effective moves and does not deal damage to itself. Hitmonchan is able to increase the effectiveness and power of its abilities.
Hitmonchan and Hitmonlee both evolve from Tyrogue at the same level but Hitmonchan's capabilities make it a better option than other evolution choices. Its speed is only slightly slower than Hitmonlee, while its defense is much higher. All other statistics are the same, but the special abilities Hitmonchan possesses make its punching moves far superior to Hitmonlee's kicks.
Hitmonchan, with its higher defense, powers up all its punches by 20 percent and zeros in on opponents for higher accuracy. Hitmonlee is capable of dealing more damage to recoil attacks, but at the same time causes greater injury to itself. Hitmonlee's lower defense and higher rate of hit point loss make it a difficult party member to sustain in battle.
The skill tree for Hitmonchan is much greater in terms of diversity and potential limit cap. It is able to learn far more TM moves and special abilities than Hitmonlee so that it is effective against more types of foes. Hitmonchan learns moves that deal additional damage throughout the battle so that its increased attack power is enhanced further.