Canon EF 17-40mm f/4 L
I would say most photographers have a go-to lens of choice. They may use a wide variety of lenses in their work, but the default lens they reach for, the one that's always in their camera bag, that's the workhorse. For me, that lens would be the Canon EF 17-40mm f/4 L.
Now I know there are lots of better lenses out there, and better wide-angle lenses as well. However, there are several reasons why I always reach for this lens first. For starters, it was the first professional quality lens ( Canon L-Series) I purchased and I was finally loving the imagery that was coming out of my camera. Next, being that it's so wide, it really forced me to get closer to my subjects and I learned to shoot wide in almost any situation. If you haven't worked with a wide angle lens before, finding an aesthetic framing can take a little practice. Lastly, it passes the adventure test: light, relatively small, and rugged.
As a side note, I shoot mostly cropped sensor cameras (only an occasional roll of film or two) so I switched out the lens hood for a longer one with better coverage. This change has made for better protection of the front element and better protection from lens flaring.
Perhaps the greatest feature of this lens is its price. At under $900, it's a great starting point for upgrading your glass.