After 18 i just have budget of one more lens and would like to put a bit of a gap between two .. i was tempted by 16 but I wanted at least one 'compact' 27 i felt was not as wide, 18 is 2nd smallest .. now I can go for a 'big' lens .. so either 56mm or some decent zoom
The 16mm 1.4 is amazing, although not cheap.