As regards angular magnification distortion, the Type 4 Naglers and the Ethos are tops. Perhaps some other ultrawides may offer less angular magnification distortion than corresponding Naglers of other types. I'm not sure, but the Pentax XWs might be pretty good in this regard. I think they publish EFSDs, so I could check. Have to get back to you on that one.

As for the most distortion-free field possible, it really depends, and of course there is rectilinear distortion to think about also. Tele Vue Plossls have some angular magnification distortion, a few percentage points, IIRC, and I was most disappointed by an 18mm UO ortho (classic) in this regard. It's field stop was a lot smaller than I expected based on its AFoV, to the point that I didn't want to use it (as I'd bought it to be an eyepiece for a small finderscope). If that eyepiece is "orthoscopic" in any sense, it must be because rectilinear distortion is low. Contrast that with the 18mm Tak ortho I had--its field stop was pretty much dead-on for the zero angular magnification distortion. Unlike the Tele Vue Plossls, which show some stretching near the edge, I couldn't actuall see distortion in any of the so-called orthoscopics, though.