It depends on conditions, what sort of riding you are doing, how hilly your routes are etc.
I seem to have settled in the low to mid 50s, in gear inches, for most of my riding, so 38x18 at Herne Hill on NYD, 38x19 at Frylands Wood at the beginning of December, which is quite a hilly course.
Anyone know the sweet spot for a sscx gear? Not too spinny preferably (I have a 4 bolt mtb crankset)