Ok, I'll tell my opinion:
'Proper' Expansions
1. Old Gods: Earlier start date, pagan play. Although non-norse pagans are bland, still great fun.
2. Sword of Islam: Muslim play and many muslim mechanics. My rating is based on the current state of decadence.
2½. (Rajas of India patch material)
3. Sons of Abraham: Religious mechanics (especially College of Cardinals), Jews and events, all nice.
4. Legacy of Rome: Retinues and Byzantium stuff. Not all that much, but not bad.
5. The Republic: Republics are different enough to feudal realms. Fun at times.
6. Rajas of India DLC Material: Yes, we can play as the Indians, but what else? I don't even remember.
This doesn't mean that RoI is bad by any stretch of the imagination, just bland.
I can't rate Sunset Invasion, since I haven't played it.
Facepacks:
1. Norse: They look nice and fit the "viking ruler", even if they don't portray, say, 15th century Swedes properly.
2. Celtic: I like them, even if some moustaches look like they are glued on.
3. Persian: Nothing wrong with them.
4. Russian: Just okay.
4 (tie). Mongol: Same as Russian.
5. Turkish: Strange gray skin.
6. African: Why do Ethiopian Miaphysites look like West African tribesmen?
7. Mediterranean: Goes without saying. Gettin' tired of them potatoes...
Other stuff
1. Dynastic coats of arms: A must for any playthrough.
2. Ruler designer: Nice for making your own character.
3. Music packs: Songs of the Caliph and Songs of the Old Gods always put you in the right mood.
4. Unit packs: Nice, but not very noticeable in the long run.
5. Customizer: Just not very much in this one, I'm afraid.
I have never used the converter, so I am unfit to say anything about it.