I haven't tried many 28mm but I do like the Leica 28mm elmarit - the rendering is nice, super sharp wide open, hardly any distortion, quite resistant against flares and color fringing. Plus its super compact. Some users say it has pleasing bokeh (if you like that kind of thing).
If you don't mind a compromise, I hear a lot of good things about the Zeiss 25mm 2.8, which is a bit wider than 28mm.
FARI