Depends on what I'm in the mood for. I'm less concerned about weight than photographic ability, as I am usually hiking for the purposes of taking photos. I usually choose one or two of these:
Sigma 10-20
Canon 17-40L
Canon 24-105L
100 Macro
Though sometimes if I'm feeling masochistic I'll lug my 100-400 around, as it is my favourite "go-to" lens for a lot of outdoor work.
In any sort of 1-lens consideration, the magnification of the lens is important, if you like taking close-ups, weather sealing if any, and IS. So of the above lenses, my first choice would always be the 24-105, as it is weather-sealed, very versatile, does good close-up work, and has IS. But it lives on my wife's camera, so that's not usually an option for me...
All good things are wild, and free.