Does it have to be only one walk-around lens?
My usual walk-around kit is Sigma 30/1.4 + Canon 85/1.8 on 40D, often with a 430ex or Metz 58 flash. If I want less bulk and lens changing - 17-85 IS - a very useful and versatile focal range with decent IQ, until I upgrade it to a 15-85 IS.
Just one walk-around prime - most likely a 60/2.8 macro - can't beat this lens, it's really good in many things, not only macro and close-up shots. Have, bur rarely use 50/1.8, as been said, it's neither wide, nor long on a crop camera, although it is capable of very good IQ. used it only 2 or 3 times for the last year or so.