A bit more in depth than just “it depends”
The civilization guides give you a little more insight to the discussion on the situational advantages of each civilization, discussing key features, their technology tree and some potential build orders. The guides are by no means exhaustive but will hopefully give you some insights into how the civilization could be played.
The Aztecs are a civilization that is considered a very strong one and has been for long, with good economy and adaptability. They have counters for everything your opponents can throw at you which makes them a wonderful civilization throughout the game.
The Berbers are a relatively new civilization that has good economy and cheaper Stable units, a great Cavalry civilization. They also have the mounted skirmisher “Genitour” that is very effective vs archers and has the mobility to chase them down.
Britons are arguably the best Archer civilization in the game with a good Dark Age economy and strong Feudal Age pressure with their faster producing Ranges, as a plus they gain extra range on their Archers in the Castle and Imperial Ages making them hard to engage.
Bulgarians have a great if not the greatest Men-at-Arms rush in the game because of the free instant upgrade of the Militia-line when they age up. They can also access the Krepost that is basically a small Castle. They are a strong Cavalry and Infantry civilization.
The Burmese have a strong economy with the free Lumber Camp upgrades. Their Infantry gain 1 attack per age and have strong Monks with half price upgrades and a team bonus revealing all the relics on the map.
The Byzantines have a great defence with the cheaper ‘trash’ units and are also strong with their cheaper Imperial Age that can lead to a great advantage in the latter stages of the game. Their unique unit the Cataphract is also super strong and are a hard unit to counter.