For me it was air law first then navigation. I found that a good sequence and it was nice to do a 'theory' type exam (boring!!) followed by a practical one (interesting!!) ie navigation.
Some pilots I have spoken to struggled and still do with nav but I never did. Don't know if this is because it was drilled into me early in my flying hours and i then had many hours practice when outside the circuit. I'm sure it helped.