When comparing them side by side, Hitmonlee would be considered the better Pokémon by a very small margin due to a higher base speed and base attack. Hitmonchan and Hitmonlee are both very powerful fighting Pokémon and share several key stats that make a Pokémon strong.
Both Pokémon share the same weaknesses and have abilities that are very similar. However, Hitmonlee has a higher base speed at 87, versus Hitmonchan at 76. Speed is one of the most important stats Pokémon can have, as it enables them to strike first and have more turns in combat. Hitmonlee also has an advantage in terms of base attack, which is 120 compared to 105 for Hitmonchan.