Best walkaround lens? The 28-300mm f/3.5-5.6L
IS. Covers moderate wide to quite long, has IS, weather sealing, and optical quality is equivalent to the 24-105mm. You can capture almost everything with just that one lens. Sure it's a little heavy, but if you want light, get a PowerShot S100. Carry it with a Blackrapid strap and the weight is on your shoulder, so it's ok to carry around for the day.
Best walkaround lens? The 24-105mm f/4L
IS. Versatile focal length, has IS, weather sealing, quite sharp. Great value when purchased in a kit with a new 5DIII (or 5DII).
Best walkaround lens? The 24-70mm f/2.8L II. Tack-sharp, weather sealed, IS not as important with shorter focal lengths, top optical quality, and f/2.8 gives shallower DoF for portraits.
Best walkaround lens? The 35mm f/1.4L
. Combined with a FF body having great high-ISO capability, you can't beat a fast, wide angle prime for walking around at night.
Best walkaround lens? The 16-35mm f/2.8L
II. Captures stunning ultrawide shots that you just can't get with a standard zoom, but at the long end it's useful for general purpose shooting. The fast aperture means good low-light capability.
FWIW, I just got back from a trip where I took the gripped 5DII, 28-300L for outdoor walkaround, 35L for indoor ambient, and 135L for portraits. I found it to be a very versatile kit. I brought a 430EX II and didn't use it at all. It all fit perfectly in a Lowepro Flipside 300 (which I enclosed in a Pelican Storm im2500 for the plane).
Only you can decide this matter based on the way you like to shoot.
Yeah, that's what I meant. But you put it much more succinctly.