Remember that Genghis had an awesome nomad army which could, if well led, easily pwn any other contemporary army. Genghis was a good commander no doubt, but his achievements must be put on perspective.

Napoleon, on the other hand, suffered reverses and was ultimately defeated, though we should remember that commanders were likely generally better at that time due to military academies and professional armies.

Alexander conquered the Persian empire, but Greeks had defeated Persians before and Alexander had a good and a well-balanced army. He didn't show any huge tactical skill, but he wasn't bad either, and we really don't know that much.

Genghis conquered the most territory, though Alexander perhaps conquered more in relation to time he spent conquering - He, after all, died before his 33th birthday.