This is one area where I feel APS-C photographers ARE literally "spoiled for choice" these days; that is there are many great options
when looking for ultra wide angle (UWA) zooms
While the OP stated that a zoom isn't necessarily needed, it was also stated that would be considered. I would suggest a zoom. I'm also an outdoor person, and landscapes are some of my favourite photos to take. (I strongly dislike fisheye!) I've felt that the flexibility of a zoom does help in composition - and a few mm either side makes a lot of difference at these ultra wide angles.
Obviously there is lens to lens variation and QC, but I've done a lot of researching - and my own real world testing of UWAs... and am providing the below as some assistance to the OP.
There are several options I'd recommend.
Tokina 11-16 f/2.8 sharp and particularly good if one enjoys low light (and particularly astro / night photography)http://www.photozone.de/canon-eos/379-tokina_1116_28_canon?start=1
Canon 10-22mm f/3.5-4.5 as a good OEM option (as Neuro suggested, a 2nd hand would fit budget)http://www.photozone.de/canon-eos/174-canon-ef-s-10-22mm-f35-45-usm-test-report--review?start=1
Sigma 10-20mm f/4.5-5.6 as a great UWA lens all round - good IQ and one of the best value. It's slightly slower than the Canon, but for most landscapes that's not a huge issuehttp://www.photozone.de/canon-eos/307-sigma-af-10-20mm-f4-56-hsm-ex-dc-lab-test-report--review?start=1
Actually what verysimplejason wrote below is incorrect
and not backed up by any data or research.
or Sigma 10-20mm F3.5 Ex Dc Hsm. Make sure it's F3.5. The other one isn't that good.
The Sigma 10-20mm f/4.5-5.6 is actually sharper
at the wide setting (and most focal lengths) than the Sigma 10-20mm f/3.5.Compare the analysis of both
Sigma 10-20mm lenses here (in Nikkor format, as Photozone hasn't reviewed the f/3.5 in APS-C Canon mount): http://www.photozone.de/nikon--nikkor-aps-c-lens-tests/467-sigma_1020_35_nikon?start=1http://www.photozone.de/nikon--nikkor-aps-c-lens-tests/308-sigma-af-10-20mm-f4-56-dc-ex-hsm-lab-test-report--review?start=1Tests (& user reviews - also those who have purchased later f/3.5 versions) at SLR report the same
, ie that the Sigma 10-20mm f/4.5-5.6 is sharper at most focal lengths (with the f/3.5 having noticeably softer corners in particular - not just the decentering issue, though to be fair some f/4.5-5.6 lenses also had decentering issues):
Sigma 10-20mm f/4.5-5.6 http://slrgear.com/reviews/showproduct.php/product/171/cat/31 being sharper than
Sigma 10-20mm f/3.5 http://slrgear.com/reviews/showproduct.php/product/1250/cat/31
The only downside of the Sigma 10-20mm f/4.5-5.6 is CA at 10mm. However thankfully automated post processing can get rid of that very well, so easily.
If I were buying lenses today, perhaps the option I would go with is the Sigma 8-16mm (the widest in its class) which has received good reviews - but a bit more expensive (so you might need to look for a 2nd half / refurbed model) http://www.photozone.de/canon-eos/515-sigma816f4556apsc?start=1http://slrgear.com/reviews/showproduct.php/product/1330/cat/31
There are other options, like the older Tokina 12-24mm f/4 (not bad) and Tamron AF 10-24mm f/3.5-4.5 (not the best), etc, etc, etc - but I'd recommend the first one's I've listed. I definitely notice the difference between 10mm and 12mm, with wider being better.
As I also have the Canon 15-85mm (love this as an all purpose lens) - I don't need / use the Sigma as much as I used to when my 'wide' was 18mm on a kit zoom. But I still do use the Sigma regularly for dedicated landscape, architecture and special effects.
I use my Sigma 10-20mm UWA lens at these settings the most: 10mm f/8 (usually at ISO100 or ISO400) where it consistently is shown to be a tad sharper than the Canon. The Sigma provides great results, I've taken thousands of photso with it. The Canon 10-22mm is a good lens - but I don't like the huge, weird lens hood and briefly a few other UWAs. When I bought my UWA, there were not the later options of eg Tokina 11-16mm f/2.8 or the Sigma 8-16mm.
Also, it is significant to note that UWA lenses for crop sensor DSLRs (ie APS-C cameras) are consistently sharper than their FF equivalents at the edges and particularly in the corners, (and especially so with lenses wide open). Even the higher end FF UWA lenses have noticeably softer edges & corners than what can be achieved on APS-C. This is actually one significant reason I'm happy to use UWAs on APS-C cameras.
As I started my post with... we are really spoiled for choice in this segment.
Best wishes for your purchase, let us know how you go!