Teutons, Vikings, Celts, Chinese, Britons, Ethiopians, and many Mesoamerican civilization such as the Aztecs, Mayans, Incas) due to their high mobility of their siege weapons and faster firing Mangudai makes it incredibly hard to deal with late game due to the lack of a mobile army that would deal with the Mongol's lategame hit-and-run tactics. The Mongols will have no problem facing civilizations with less mobile army compositions and/or weaker cavalry (i.e. Saracens and Indians) to deal with the Mongol's frontline cavalry. Berbers' Camel Archers and Genitours, Goths' Huskarls, and Italians' Genoese Crossbowman) or civilizations with very strong camels (i.e. Gameplay wise, the Mongols will endure a hard time against civilizations that either have strong anti-cavalry archer capacity (i.e. Mongols can also rely on Camels, being, beside the Chinese, one of the two only civilizations not coming from Africa or the Middle East to possess Camels, compensating for their lack of effective heavy cavalry. However their defense is somewhat lacking, as their Watch Towers can only be upgraded once and cannot build Bombard Towers. Overall, the Mongols excel at mobility and quick devastation. Mangonels and Scorpions become unusually deadly as well. With their Drills upgrade, a small group of Mongol siege rams can perform hit-and-run attacks on isolated bases and resource collecting operations.

This means that Mongol games often include at least some (if not hordes) of Cavalry Archers and Mangudai along with siege rams filled with champions. They also have complete access to all regular infantry technologies with the exception of Halberdiers and all siege weapons except for the Bombard Cannon can be upgraded to their maximum level. They have nearly every upgrade available to them or at least more than any other civilization. A fully upgraded castle in the Imperial age has like 8100 hitpoints. Their buildings automatically get stronger each time you age up. If you like defensive civs then try the Byzantines. The Mongols' greatest advantages lie in their cavalry archers and their siege weaponry. As iEuro mentioned there is no best civilization.

Aoe 2 best civ