The Roman Empire continued from 201 BC to 47 CE, covering half of Western Europe and the UK, including the Mediterranean. Meanwhile, the Mongol empire continued from 1206 AD to 1386 AD, conquering the majority of Russia, Islam and Slavic people. Their conquest was rapid and cruel, often killing thousands of people behind them and succeeding in the civilization which conquered the majority of the land. Both empires were very successful, but in my opinion, the Roman Empire lasted longer and was more successful as it was ruled by many successful and powerful leaders like Julius Caesar and Alexander the Great Did.
The Mongol empire is prevalent, but in consideration of time, it is the greatest and most powerful empire in history. The Mongol empire survived only 185 years and was small compared to the Roman Empire, but in such a short period of time it was able to satisfy more hope than many countries, empires or kingdoms. The height of the empire covers all areas of the land between modern Korea, China, Russia, the Middle East, India and between them. But in many ways the greatness of the Mongol empire will eventually destroy it.
Today: Japan (the only remaining emperor in the world). History: Roman Empire, Byzantine Empire, First and Second Bulgarian Empire, Serbian Empire, St. Roman Empire, Russian Empire, Korean Empire, Mongol Empire, China Empire, 1st and 2nd French Empire, Austrian Empire, 1st Mexican Empire, Brazil Empire, the German Empire (not left in Europe since 1918), Emperor of India (In India in 1947 India was no longer used after independence of the British Empire). German Champion Caesar and Bulgaria / Serbian Champions Tour are coming from Latin Caesar which means the emperor. One of the titles of the Sultan of the Ottoman Empire is rough transliteration of Osman Turkish by Kaysar-i-Rûm (Emperor of Rome) and Kaysara by Caesar (Emperor). German Kaiser's Kaisar-i-Hind is the Urdu translation of "Indian Emperor".
After the collapse of Constantinople in 1453 and the Roman Empire after the collapse of the 1806 Roman Empire, the title of "Emperor" circulated widely among the monarchs of Europe. As the Austrian Habsburg dynasty tried to govern Germany under their rule, the Austrian empire claimed to be the successor to the Holy Roman Empire. Founded in 1871, the German empire also claimed to be the successor of Rome through the lineage of the Roman Empire. Both empires used the royal title Kaiser (from the Latin "Caesar"), the German emperor. The successor of the Austrian Empire, the German Empire, Austria - Hungary will fall into the aftermath of the First World War between Russia and the Ottoman Empire. Both are claimed to be inherited by the Eastern Roman Empire.