There is no such thing as the ultimate landscape lens.
I have never really liked the distortion on the Ef-s 15-85 mm from 15-20mm. And generally changed to my Sigma 10-20mm f4-5.6 when going that wide. That said I prefer 15-55 for pure landscape. Unless there is something in the foreground to act as the subject of the seen I rarely go wider. I have used a 70-200mm for landscape when there was something at a distance that was interesting. For the most point I consider landscapes with an ultra wide and no subject "Ultra-wide abuse". In general the landscape is so small that the hole thing becomes boring. Especially with a large empty foreground.
That said some of my favorite landscape pictures have been taken with an Ultra-wide with a subject in the foreground. Such as a tree growing on the side of the Grand Canon, wave crashing into a huge rock with the sun setting in the background. I think that the best Ultra-wide IQ wise on a crop is the Tokina 11-16 f2.8. The canon 11-22 is more versatile and holds its value better. The Tokina is really a specialty lens with a limited focus range.
I bought the Sigma 10-20mm f4-5.6 after extensive review on a site that compared the canon, Tamron, and both sigma 10-20mm's. The site had full size images at a variety of focal lenght and through the entire f stop range of the lens. To me visual the canon was a little better IQ that the Sigma f4-5.6. The Sigma f4-5.6 was better stopped down than the Sigma f3.5. All of them were better than the Tamron. They were all extremely close. So it is hard to go wrong with any of them. The Tokina however stands out for the f2.8 but was not in the review. I could not justify the priced difference between the Canon and the Sigma to buy the Canon. Also 10mm vs 11mm is a big difference. I was vary happy with the sigma until it went for a swim with my old 60D.
The biggest thing with the Sigma is that it focus different than all of my other lens. It works best if you turn on all focus points. If you use the center focus or an outside edge focus point then it will stop trying to focus the second it a achieves lock on the center point. The dept of field is such that it may be at the edge of the dept of field. This means that using center only could lead to soft out of focus corners. Using a edge focus point could lead to a soft out of focus center. I do not think Af micro adjustments could fix something like this. It is more of a function of the focal range/dept of field and sight lens distortion. But once you figure this out it can easily be accounted for.
But if I had not gone full frame and had not bought the Nex6 and a 10-18 F4 I would be buying a Tokina to replace my Sigma that is beyond reasonable repair.
6D, 60D, Rebel XS, EF-S 15-85, EF 70-200f4L, EF 100f2.8L, Tamron 70-300VC, Tamron 28-75f2.8, Tamron 150-600, EF 40f2.8, EF 50f1.8, EF 85f1.8, EF 24mmf2.8IS EOS M 22f2, 18-55, EOS M-EF adapter, Nex6, A7II, SEL50f18, SEL1018, DN 30f2.8, adpaters and Fd and M42 lens.