I've got, or have had, a number of 20mm or sub-20mm lenses, including the M 20/4, A 20/2.8, FA 20-35, Tokina 17/3.5, Vivitar 13mm 2.8, Sigma 15-30, Sigma 20-40, Sigma 12-24.
A summary of my experiences:
I'm keeping the M 20/4 for its portability & flare resistance. Its corner sharpness isn't great though.
I'm keeping the A 20/2.8 too. Its corner sharpness is a bit better than the M 20, but it's field curvature seems worse, but it's easy to deal with. It's bigger than the M but A metering helps.
I'm also keeping the FA 20-35. I keeping this as its corners are better than the two MF primes and it can also cover up to 35mm and of course it's AF too. It's a bit bigger than the primes but it's still fairly light.
I'm selling the Tokina 17/3.5. Its sharper on the edges & corners than the M20 & A20 but its flare resistance is worse and I don't like the colours as much.
I'm keeping the Sigma 12-24 (at least for the moment). It's big and heavy but it's super wide. The Sigma 8-16 is a better performer on APS-C (I have one of those too
) but it's as wide as I have that can cover the full frame - so it stays.
The Sigma 15-30 was a bit sharper than the 12-24 but FoV trumped sharpness for me so the 12-24 stayed and the 15-30 went.
The Vivitar was better optically than the 12-24 (apart from the moustache distortion) but the Sigma 12-24 covered that focal length and more, and I'm eyeing off a Laowa 12mm, so it went too.
Ultimately each of their lenses have/had their own individual characteristics and what was right for me might not have been right for someone else. I tend to favour things like FoV, portability, manual focus feel, flare resistance and build construction over things like corner-to-corner sharpness, but YMMV.