It’s a really exciting period full of interesting characters to play as, like Alfred the Great, who historically founded the Kingdom of England, but has to usurp his useless brother Aethelred first. With The Old Gods installed players can choose to begin the game in 867, when Christianity was less dominant and Vikings prowled the seas. The biggest change however, and the reason this remains a top tier DLC even if you have no intention of playing as a pagan, is the addition of a new start date.
This also makes a great goal for a fun playthrough. To solve this an ambitious player can ‘reform’ a pagan faith into an organised religion, making it more stable and able to compete on a level playing field with Christianity and Islam. Pagans are very strong in the early game, where they can amass large armies and raid their neighbours, but have huge problems keeping their realms together as history marches on. Paganism is the focus here, but it comes in several different flavours, from the familiar Vikings to West African tribes. The Old Gods does both, which is why it is hands down the best expansion, and the first you should buy.
Some are more universal and add interesting things for all players. Most Crusader Kings 2 DLCs focus on a specific religion, culture or government type and are mandatory if you want to play as that kind of empire.
