Let's see...
Ruler Designer lets you custom-design the ruler you want to start with at the beginning of the game, although fair warning: You lose all the alliances that the original character would have had, and you can't use Ruler Designer in Ironman mode (which is the only way to get achievements). It's also not absurdly cheaty, because if you pile up the negative traits to take a lot of positive traits, first you'll be hated for all the negative traits (I once did that to such a degree that the entire kingdom I started in rose up and overthrew me in the first couple years), and second that ruler will eventually die.
Sword of Islam basically just allows you to play Muslim rulers. They have different mechanics than Christian rulers, but despite buying the DLC I still haven't played them myself. Whether this is worth it depends on whether you want to play muslims or not (or whether you just want to get all the DLC when it's on sale or whatever).
Legacy of Rome adds retinues (useful no matter who you play), and the ability to restore the roman empire and mend the great schism (between catholicism and orthodox christianity), and some decisions and events and stuff if you're playing the Byzantines. I'd say this is worth it.
The Republic makes merchant republics playable. They're fairly interesting. You play as the head of a merchant family. The families run for election for leadership of the republic, and even if you lose the election (unlike a regular nation), you don't lose the game, since you still have your family holdings.
The Old Gods lets you start at 867, makes pagans and zoroastrians playable, and gives revolts leaders. Definitely good.
Sons of Abraham, Rajas of India, and Charlemagne are ones that I don't have, so perhaps someone who does have them can
There's also Sunset Invasion in case you want to see Western Europe invaded by the Aztecs.