The SMC Pentax-M 28/2.8 is small and is a decent lens.
The SMC Pentax 28/3.5 is larger and slower, but is maybe a bit better optically.
The SMC Pentax 24/2.8 is a very good 24mm
I second Colton's suggestions. If you want small and affordable, the M28/f2.8 is hard to go pass
The SMC-K 28/f3.5 has excellent image quality but it's slow (which also means dim VF) and it's reputation is pushing prices up.
The SMC-K 24/f2.8 is great, but more expensive that I think it deserves.
If money isn't an issue and you have the time to search, the SMC-M 28/f2 is one of the best wides I've used.
All mentioned lenses above I own and have used (however on digital ) but the M 2.8/28mm and agree with all assessments ( though my Pentax K f2.8/24mm did not cost me much. )
I do have the Pentax-A f2.8/28mm which some say is equal to the M lens, other say it's not ( see more below ) and I find it to be excellent! ( Aperture ring on my A lens is much less smooth than those of any other, earlier made lens by Pentax. It's the only Pentax A lens I own, therefore I can't be certain but that made me believe that, because on an A lens aperture can be set on the camera, Pentax saved on the making of the manual aperture ring )
Extensive lens reviews at Pentaxforums,
of the Pentax M 2.8/28:
https://www.pentaxforums.com/lensreviews/SMC-Pentax-M-28mm-F2.8-Lens.html
of the Pentax-A 2.8/28:
https://www.pentaxforums.com/lensreviews/SMC-Pentax-A-28mm-F2.8-Lens.html
Some say they are optically identical, others say the A is quite superior, here quite some disagreement on how they compare
😉 https://www.flickr.com/groups/40939305@N00/discuss/72157624640805669/
The f2/28mm Pentax lenses are said to be fantastic, of the highest rated lenses they are pricey
https://www.pentaxforums.com/lensreviews/SMC-Pentax-M-28mm-F2-Lens.html
even higher rated, most likely also even rarer and more expensive again, also bigger and heavier is the Pentax K f2/28mm. There are a number of articles on the net that the say that it is a Zeiss Distagon clone.
For a cheaper alternative recommendable Vivitar f2/28mm. Mine is the one made by Kiron and on this infinity test of mine on Sony A7 it fares very well, both center and outer corner:
https://www.flickr.com/photos/kuuanslenstests/collections/72157670043721223/
though not tested, at least by me, same line Vivitar f2/24mm made by Kiron is said to be very good too
( there are also Vivitar f2/24mm and f2/28mm made by Komine. Komine is said to have made the Series 1 lenses for Vivitar and said to be superior, also smaller. The Kiron made have a floating element and are difficult to service. I do not have experience with the ones made by Komine. Serial numbers of those made by Komine will start with 28, those made by Kiron with 22 )