For a single walkabout FF lens, I think it's hard to beat the 24-105. Good range, and IS is nice to compensate for f4. I often pop the 50/1.4 in the bag to handle low light needs since it's pretty light and compact.
When I'm willing to haul more weight and bulk, I leave the 24-105 at home and go with the 16-35, 50, and 70-200.

