I have no definite answer - just not for myself. But ...
1) I like round about 40mm (equiv to the 24mm on a crop body) for landscape, towns, nature ...
2) I like round about 100mm (equiv to the 60mm on a crop body) for landscape, towns, nature, macro, people, architecture, etc. - for me 100mm are the standard focal length.
If I had to choose just one lens: I would go for a 100mm (macro).
If I had to choose two lenses: I would go for the f/2.0 100mm and a f/2.0 40mm pancake.
If I had to choose three lenses: The three lenses mentioned above because the f/2.0 100 doesn't attract any attention, 40 + 100mm is lightweight and compact and a macro for special tasks.
Or the bottom line: It depends on each ones photographic style which lens is "THE" walkaround lens. With my 40D I like to use just the old EF 24mm/2.8 and the EF-S 60mm/2.8
as a light, compact and very versatile walkaround set. Sometimes two 40Ds with both lenses mounted - balances the weight on left and right shoulder
Best - Michael