Of the ones that I've tried (Canon 28mm f/2.8 IS, Nikon Nikkor 28mm f/2.8 AI-s, Contax Zeiss Distagon 28mm f/2.8 C/Y, Canon FDn 28mm f/2.8...), my favorite is the SMC Pentax 28mm f/3.5 (original K, non M version.) It's a slow lens, but I shoot landscapes, so my interest is more stopped down performance (although it does quite well at larger apertures.) I'd love to have the Leica R 28mm f/2.8 Elmarit V2 lens that Rich and a couple others mention, as well as the new FE 28mm f/2. But there is something about the Pentax that makes me think I'd keep it no matter what. It gives such a bang for the buck, it's amazing.
Here's a f/11 shot with the a7R.