If it's just a general wander, I'll probably have the 18-70 fitted. If I'm expecting to need something longer, the 70-300 VR is more likely. I have been known to carry 2 bodies so I have both bases covered, although I'll almost always leave one in the bag. IF I can talk my wife into it, there may also be another body, this time a film one (for FF) with a 12-24 Sigma attached. If I'm lucky, she'll be carrying the 8mm fisheye as well, just in case.
Chances are that if I'm out with a DSLR, I'll have the rucksack on my back, so I'll have 12-300 covered, possibly with a 1.5x telecon as well. Don't often carry the 170-500 unless I'm sure I'll want it - a 'pod of some sort is almost a must with that!