I chose Hitmonlee because his design looked cooler to me than that of Hitmochan and the attack stat is more important to me than the defense one. Plus, the Pokemon's appearance in the anime is absolutely kickass.
Hitmonchan his design to me looked superior, or perhaps the boxing gloves did it for me. I was also rather found of the different element punches he could perform.
In Generation I, I chose Hitmonchan. Now in Generation IV, I raise a Hitmonlee, teach it Blaze Kick, Hi Jump Kick, Mega Kick, and Rolling Kick, breed it, and then evolve it into a Hitmonchan that knows kicking moves