My Canon 10-22 (digital only) is lots of fun. I'm sure it's quite similar to blary54's Tokina on the wide end, although lacking in the constant fast aperture department. Architecture and landscapes are obvious targets, but this lens is harder to master than one might think for those kinds of photos. The real fun for me is getting in close with the ultrawide--interiors and objects--the possibilities are endless.
For every day, my old, discontinued Canon 17-85 is the handiest one I own for a one-lens outing. It takes great photos, despite some criticisms you'll see on some forums.
Over the years, stretching far before the digital age, I've collected only 3 primes, but primes are best in so many ways. There should be a law requiring every Canon owner to buy a 50 mm/1.8. I paid about $50 many years ago, and it's still going strong; I think it's about $100 now. Also in the film era, I picked up an 85 mm/1.8, which continues to do a great job on portraits with the APC-1.6 sensor. Finally, I got a 28 mm/1.8 to replace the 50 as a "normal" lens for the smaller sensor. In addition to perfect color and sharpness, along with the speed, it makes the camera light enough to carry all day without strain.
(As much as I'd like to own the entire collection of "L" lenses, the weight and cost just don't seem worth it for the way I shoot.)
I love the smell of Proraso in the morning!