175/50 the speedo over-reads massively, and even on stockish height suspension I found compromised ground clearance too much. I used them because Yoko A539s were dead easy to get hold of in that size!
Have also run 185/60 in the past, which were okay - but a bit like balloons. If I couldn't get a tyre I liked in 175/60 I'd go for the 185/60 instead.