I don't know which Minolta 20mm everyone here had been using, but mine back in 1986 was - and remained for the 15 years or so I had it - extremely sharp wide open, and a good match for the Biogon. Admittedly I only ever had the Biogon for any length of time in Contax G form, which is supposed to be better than the reflex Biogon. The Minolta was also much better (by far) than the Leitz 19mm f2.8.
The Minolta 20mm with a bad reputation is the MD 20mm f2.8, which was redesigned from the earlier (unrivalled, superb) MC 21mm f2.8. The 21mm f2.8 had internal floating elements, a Distagon-type design, and a 72mm front thread. It had superb full aperture performance, very good close up flatness of field, and very even illumination thanks to the generous design in terms of optical real estate. The 20mm f2.8 MD suddenly shrunk the lens to fit a 55mm filter thread and went 'Pentax size' (or Olympus size) in proportions. The result was a lens with severe moustache-type distortion, strong vignetting to the corners, and poor full aperture sharpness. Professional Minolta users, myself included, criticised it heavily. Most switched to using the 17mm f4 instead, despite the loss of speed and the higher distortion levels than the old 21mm. It was essentially a traditional large retrofocus design uncompromised by attempts to miniaturise.
In 1985 - only five years after the MD design 'replaced' the big old 21mm - Minolta unveiled the 20mm f2.8 AF and everyone breathed a sigh of relief. The good old Distagon 'bucket' style design was back in; internal floating group to correct close focus; large front element for superior illumination; low distortion; back to 72mm filters. It tested out as a fair match for the old 21mm, putting Minolta back on top with one of the best faster aperture wides on the market.
It was, also, part of a set of lenses which used coatings and glass types to give almost perfect colour balanced transmission (with 3CC/LB units) from 16mm to 500mm - combined with matched contrast. The AF lens system from 1985/6 was the first and possibly only still camera lens system built to the same colour/contrast consistency as professional movie lenses (where colour shifts from scene to scene were unacceptable). Minolta started doing this around 1975, when the XE-1 was launched and used the first shutter/transport mechanism to be compatible with Wess pin register mounts without requiring a pin register camera. We used Minolta for audiovisual production exclusively because it allowed animated sequences in the field. Wear and tear may make the XE-1/XE-5 with the Copal Leitz CLS Shutter less accurate if bought today, but in its time this was the most accurate shutter ever made, and the only one which spaced frames precisely.
That is the sort of heritage the 20mm came from after the 'blip' in Minolta design philosophy represented by the X-700 line (an attempt to match Canon's synthetic resin body and lens production introduced with the AE-1). The 9000 AF and the entire 1st generation lens line were an attempt to return to the values of 1975. I think, in the lens range, they almost succeeded in some respects, and outdid 1975 in others.
Sony had decided to continue making the 20mm f2.8 AF lens, which is pretty remarkable - 22 years down the line after its first announcement.
I've tested the Sigma 20mm f1.8, 24mm f1.8, 18mm f3.5. Tamron do not make now, and have never made, a 20mm f2.8 though they did have a 20-40mm zoom. None of these come anywhere close to the quality of the Minolta 20mm f2.8.
David