Crusader Kings II: Sunset Invasion Review (KERNBOOM)
The DLCs in a nutshell, and in order of my opinion of necessity:
More cultures / governments / religions available to play:
Charlemagne (earliest start date, more cultures)
Sword of Islam (Islamic cultures, Iqtar, Islamic government / religion)
Old Gods (Vikings + new start date)
Horse Lords (Nomads, Mongol Hordes, Silk Road)
Rajas of India (Hinduism, Buddhism)
Legacy of Rome (Byzantine, retinue's, factions)
The Republic (Merchant republics)
Jade Dragon (China, but hardly)
Sunset Invasion (Aztec, but even less so than China)
More actions / events while you play:
Way of Life (form your character)
Holy Fury (more control over religious aspects and wars)
Monks and Mystics (adds secret societies and items)
Conclave (council votes on your actions)
Sons of Abraham (Abrahamic religious options)
The Reaper's Due (adds court physician, illnesses and hospitals)
Sunset Invasion adds the Aztec, but they are off the map, and will invade Europe at a set or random starting date (your choice). You can become them and that's most of the fun, next to fending them off or laughing at Spain with tears of sweet karma. You can totally miss out on this DLC and all you will miss is the sudden invasion of an overpowered empire. I keep this DLC active when I play but I can't recommend it among the others.