I beleive, after passing them all, that how hard they are depends on your skill set.

Definately take FAR first.  Than an easier one to squeeze it into that same testing window.  I did Hard + Easy each window, passing all the first time.

Which ones are easier?  If you are math orientated (great at number crunching), then take BEC.  If you are conceptually orientated (do well with concepts), then AUD and REG will be easier.  I passed AUD with a 90% after 1 month of study - it's mostly conceptual.