Though I hate the idea of having to be limited to just one lens, I will vote as follows:
Crop: EF-S Canon 15-85
FF: Canon EF 24-105
Both of these have been my trusted pick-one-for-this-thip lenses, as they cover a very useful range and perform well (if not brilliantly).
Pretty boring, I know. My favourite lens is by far the Canon 70-200 f/4.0 IS, but that would not do it alone.
PS: I have no experience with the much praised 24-70 f/2.8 II, so I did not consider that for this poll.