For a single lens option that goes from moderately wide to short telephoto, look at:
EF-S 17-85 IS
pros: IS - its like traveling with a monopod, minus the hassles.
Range is quite good (28-135mm FF equiv).
cons: barrel distortion is quite pronounced at 17mm. It will show up if you are shooting straight lines like architecture. Not really a problem for portraits or nature.
EF 17-40 L
Pros: Outstanding build quality
It's a full frame lens.
Cons: image quality is only slightly improved over the 17-85
Still have some barrel distortion at 17mm.
Limited range (up to 64mm FF equiv - not quite up to traditional portrait length of 80-135mm)
EF-S 17-55 IS
Pros: Reputed to be the sharpest lens in its class for 1.6 crop
IS - see note above
Fast F2.8 constant aperture
Cons: Cost
Weight
Sigma 17-70
I don't know anything about this lens. But you can do your own research for it.
Quite frankly, traveling with only one lens is going to be a compromise and it depends on what you think is most important to you: is it cost, image quality, range, IS, or maybe the ability to share the lens with a FF camera. It is never going to be wide enough, or long enough, or fast enough. For me, the range and IS of the 17-85 won me over. If I go with only one lens, this is my choice.