I find for my photography the one lens I choose most often has been the TS-E 17mm f/4L tilt-shift lens. It's been my favorite lens since the day I got it.
Allows for landscape, automotive, cityscape, all kinds of scapes... Plus tons of creative possibilities with both shift and tilt functions. Further, it's one of the most challenging lenses to learn to use properly to its full capability. Love that about it! I can't imagine ever parting with this lens.
Sammael on the Town by
ddk632, on Flickr
Las Olas River House Condo Tower by
ddk632, on Flickr
Starry Night on a Mountain Road by Canon EOS 6D by
ddk632, on Flickr
Bayside Miami Architectural by
ddk632, on Flickr
Rickenbacker Night Shift Panorama by
ddk632, on Flickr
Christian Science Church, Boston by
ddk632, on Flickr
Beyond the Sky by
ddk632, on Flickr
Although I also have many landscape shots at 24mm and one of these days will add the TS-E 24mm f/3.5L to the gear bag.