Alexander the Great...HAHAHAHAHAHA. He destroyed Persia but only went as far as Afghanistan (Porous), and then they were scared to go past India or present day Pakistan. They managed to also get Egypt but didn't expand any further. After they retreated Macodonia Greece collapsed.
-India's biggest Empire was Sri Vigyan Empire which was all of India, parts of Iran, parts of Southern CAR, Indonesia. Check the site below for more info...
http://www.geocities.com/raqta24/ind6.htm
-Ancient Persia at its prosperous day was from parts of Lybia, Egypt, Afghanistan, almost all of CAR, Western India (present day
Baluchistan, parts of J%K, and entire NWFP.
-Rome was close to dominating Europe but it collapsed when the Germans, and Muslims attacked them. This empire was most of Western Europe, and Northern Africa, which is bigger then Ancient Greece.
-The Mongolian Empire was the biggest in the world, it had all of Asia including Russia under its control. The only nations that weren't captured was Japan, and many say that Sri Lanka and parts of Kerela India wasn't captured. The ruler is the infamous Genghis Khan, note that he wasn't muslim.
-The Islamic empire came after that, the Arabs invaded all of middle east, then the Turks defeated the Greeks. Note that the Turks origninally came from Mongolia and CAR, the Mongol Empire was breaking up at this time. One of Genghis Khan's son converted to muslim, and this is where history is all confusing. To me it looks like the Mongol muslims helped the Arabs destroy Persia, then the converted Persian came along with them invaded Afghanistan, Pakistan, India, parts of China, CAR, and went West invading all of Northern Africa, parts of Spain, parts of Greece. In the end the Islamic Empire (or Mongol Islam empire or Arab Empire... don't know which one was leading), it had most of the Indian Subcontinent, all of middle east, northern africa, parts of Southern Europe (Spain, southern Italy, most of Yugolavia, S. Eastern Greece), Indonesia, Malaysia, parts of Philipines, and Mayanar.
Therefore the Empire that conquered the World was the Mongolian Empire because they managed to take over all of Asia, and then the next was Islamic empire. The Christian nation that conquered the world was Britain.