CKII is an absolutely fantastic game, and I strongly recommend it to anyone with an interest in strategy games.
Here's a quick rundown of the important DLC:
Sword of Islam - Allows controlling Muslim rulers. If you're interested in those realms it's worth it. If you're never gonna play a Muslim it doesn't add anything though
Legacy of Rome - Focused on improvements to the Byzantine Empire and the Orthodox Church. It's considered a must due to unlocking retinues (for you and the AI), an important source of troops
The Republic - Allows players to play as merchant republics. Like Sword of Islam, doesn't add anything if you're not playing those factions
The Old Gods - Unlocks Pagans, an 867 start date (vanilla is 1066 to 1337, with the game ending in 1453), landless adventurers, and a better rebellion system. Generally considered a must due to the gameplay features it unlocks
All the other DLC (bar Sunset Invasion, which adds an Aztec invasion of Europe, and the Ruler Designer, which does just what it says on the tin) is either cosmetics or music and thus not essential.