As others have said it depends mostly on what you are interested in playing. If you have no interest in ever playing as a Muslim character, there's no reason to get Sword of Islam. Likewise for Rajas of India if you don't want to play as an Indian, or The Republic if you don't want to play as a merchant prince. If you do want to play as some/all of these, then get the relevant DLC. Those three are all extremely modular in that respect, which is a good thing.
Buy Sunset Invasion if you want to fight fantasy Aztecs in Western Europe. That's all it adds, so again, it should be an easy decision as to whether or not that sort of thing appeals to you.
The Old Gods unlocks pagans/Zoroastrians, but also does other things (e.g. unlocks the 867 start date, and I believe it also changes rebels somewhat). Likewise, Sons of Abraham adds various events, decisions and the College of Cardinals in addition to unlocking Jews. Legacy of Rome adds retinues, as well as a few events for Byzantines (and I believe the ability for Greeks to castrate/blind people is also tied to the DLC).
Facepacks, units and music are all strictly cosmetic. Up to you whether you want any of them (although I agree that the Mediterranean facepack is significantly worse than the other facepacks).
Ultimately, it's up to you. I'd consider the most skippable to be: whichever of Muslims, Indians or Republics you have no interest in ever playing, followed by Sunset Invasion (if ahistorical Aztec invasions aren't your thing) and Legacy of Rome. Sons of Abraham is probably the next most skippable. But it's a good sale, so you might want to take advantage of it.