Hey LeBii, akin to kacpe3, I firmly believe that Common Sense and Art of War are essential, and maybe also Res Republica. For instance, Art of War gives you the opportunity to turn your vassals into Marches (a pretty neat trick if you want my opinion), and first and foremost, it grants you with the power to give objectives to allies and subjects - I cannot stress how very important that feature is. Common Sense is good value too, especially when you consider the feature that allows you to develop a province with monarch power, the various parliamentary mechanics are real fun too. Res Republica is also a favourite of mine mostly because it facilitates achievements with merchant republics.
The other DLCs are fun too but more situational and depend both on your playstyle and the affinities you have with regions of the world (fond of Japan and China I've been eye-balling the Mandate of Heaven DLC for a while), just wait for sales if you want to get them for cheap! Enjoy playing and catch you around!