Actually even though i admire the military prowess of "Temüjin" or "Genghis Khan" as most people inaccurately refer to him as. This was not his name, this was merely his title after forming the Khanate (Khan is a Title meaning leader and not a surname or family/sub tribe).
We cannot compare "Temüjin" to the likes of "Alexander THE GREAT" purely because they were two different breeds of animal. Alexander had a huge civilising influence over his realm whilst "Temüjin the Great Khan" had an entirely opposite effect.
Many Muslims forget that he almost wiped out the former Islamic Empire, whilst Alexander's belief was one of tolerance and integration of conquered lands into the Macedonian way of life.
I respect the marvolous achivements of "Temüjin" but in my humble opinion Alexander was the better General, not based on land mass or kill count but on the overall affect and legacy he left behind.
Don't take my word for it:
V28N1 - Alexander and Genghis Khan: Two World Conquerors Compared
To Quote Plutarch's (AD 46-126):