Lots of good advice here. I've tested many wide angles on my 5x7 and found that the multicoated Nikon 90/8 SW was one of the sharpest all the way to the edges. It's relatively small, using a 67mm filter, and has a 235mm +/- image circle. It's not very expensive nor really big, so that would be my top choice for 5x7 on the ultrawide end. It's roughly equivalent to an 18mm lens.
Going up a bit to a 21mm equivalent, the Fujinon multicoated 105/'8 NSW (outside lettering) is bigger and heavier, but has a 250mm + image circle. It uses 77mm filters, so it's larger, no question. Remember, though, that saving a few ounces of lens weight can be a bit of a fetish as by far most of the bulk and weight of a 5x7 outfit is the tripod, backpack, film holders and maybe the camera. A few extra ounces are a drop in a bucket.
The 120mm Angulon is small and good enough, if you can find a decent one that was properly assembled. Mine is, but I have seen repeated references to uneven Schneider quality control back when most of the Angulons were made. It's a good lens for a super-light field unit, but the Nikon 90/8 is a better lens. It's roughly equivalent to about 24mm on 5x7, with very limited movement. Y ou might prefer a later model that is factory-coated, but this Dagor-derived lens has good resistance to flare in any event due to its minimal number of air-glass surfaces.
The original single-coated 135/5.6 Fujinon W, inside writing on lens retaining ring, has an 80 degree field of view and coverage that permits a bit of movement on 5x7. I have three, used in different rigs, and each of them has been sharp. Given their low prices, Fujinon's good quality control, and small size, they're a very worthwhile consideration. They're equivalent to about a 28nn lens on 5x7. Single-coating is not a problem.
Finally, the later multicoated 150/5.6 Fujinon NW lens, outside writing on lens barrel, have been excellently sharp and great all-around lenses in my experience. They're likewise small, light, very sharp, and with good contrast. I have two and they are among my best lenses. At least when I bought mine, prices were very reasonable. It's equivalent to a 30mm or so lens, only slightly longer than the 135W but with a slightly smaller 76 degree angle of coverage, thus providing about the same level of coverage and movement due to its slightly longer focal length.
While some have urged getting older sentimental favorites that are lenses basically intended as wide-angles covering 8x10, you really don't need that level of coverage and will probably be happier with modern, high-grade lenses like the Nikkor 90/8 or one of the 135mm or 150mm Fujinons or a combination of them. A few extra ounces will not make any noticeable difference in practice and within reason should be subordinated to other, more practical characteristics such as image quality, coverage, and shutter condition.
I suggest this despite usually carrying at least one century-old Zeiss Protar VIIa set with me when in the field, in case I come across a subject that really benefits from that old, uncoated lens. By and large, though, modern lenses will likely be both easier to find, more affordable, and more versatile and satisfactory in routine practice.
