Tokina 11-16mm, f/2.8 or Tokina 11-20 f/2.8 for crop frame camera.
Shooting 135 format SLR, I discovered very long ago that 28mm was simply not wide enough for my needs, so I sold the 28mm and bought a 24mm. I once travelled to Europe with only a 24mm Perspective Control lens on the camera as an experiment, and discovered that I was fully happy with just the one lens! Eventually I got 20mm yet found that I seldom pulled it out of the bag.
Fast forward to crop dSLR use, and I got the Canon 17-85mm and found it not wide enough for me once again. So I got the Canon 10-22mm but found it a bit slow for my preferences. I traded to the Tokina 11-16mm and found that to be satisfactory, but I found myself needing to swap lenses a bit too often; so after Tokina launched the 11-20mm f/2.8 I traded to that. Now fully happy.