While I do not yet have a large, calibrated monitor for viewing and evaluating, I am happy thus far with images I have shot with Nikon-fit Tokina 17mm 3.5 lenses. (One AF and one MF, now discontinued.) The current Tokina ultra-wide zooms have been well-reviewed at several sites. I elected to purchase the Canon 10-22mm EF-S, for my Canon DSLRs, but seriously considered the Tokina 11-16mm, and as I enjoy this type of lens so much, may well try the Tokina eventually.
While it is not ultra-wide, our Tokina 100mm 2.8 macro lens is very sharp and clear, with less apparent distortion than my Canon 100mm 2.8L Macro! The out-of-focus areas are beautifully rendered. Tokina does make some really good lenses; one must, of course, look for reviews to see which are the good ones.
Canon 7D/5D/40D/1D2N; Nikon F6, D700, FM3A, & Coolpix A; Canon 40mm 2.8 STM, 135L, 50L, 35L, 50mm 1.8 I, 100mm 2.8L Macro, 10-22mm EF-S, 28-135 EF, 400mm 5.6L; Nikkor 50mm 1.2 AI-S, 50mm 1.4G, 50mm 1.8D, 16mm 2.8D Fisheye, 180mm 2.8D, 100-300mm 5.6 AI-S, 18mm 2.8D, Voigtlander 90mm f/3.5 SL II