Well it depends on what your interests are in.
The Earth Fact Book, despite its controversial elements, is pretty good for those games that will have a human-centric focus. It has human prestige classes, equipment, cybernetics, vehicles and starships. In terms of campaign useful-ness it ranks as number one.
The Racial Fact Books are pretty good. In terms of games I would run, Centarui is more important than Minbari cause it would take a very good character concept for me to allow a Minbari into the group. Centauri are much more likely to be around than the Xenophobic, self-absorbed Minbari would.
The Season books are interesting for some of the stats they provide - for instance Season 3 has the Ranger Prestige Class and Shadow Vessels. However, they aren't the top of my list of overall "needs" but nifty to have.
So in terms of my own personal tastes this is my list, from most to least useful...
1. B5 Core book
2. Earth Alliance Fact Book
3. Centauri Republic Fact Book
4. Minbari Federation Fact Book
5. Point of No Return (Season 3)
6. Coming of Shadows (Season 2)