Never give me such fatally open-ended parameters when asking for a lens recommendation.
I say the
Contax Zeiss Distagon T* 28mm f/2.8--go for an AE Japanese model if you want rock-bottom prices (vs. MM or German-made). You can typically find 'em on the used market for about $200-$250 bucks, and adapter rings are in the $30-$100 range, depending on how fancy you want to get. Bonus--it's normal on a crop.
You lose autofocus, and you can only shoot with stop-down metering in M or Av, and it's not fast enough really for available light shooting, and it exhibits a tiny bit of barrel distortion. Also, its performance wider than f/4 is not so terrific, but once it's stopped down to f/8--omg,
you don't need to sharpen.
Sharp enough to cut yourself. But not, I hasten to add, at a per-pixel level in the way most folks judge sharpness. With Zeiss, it's more about the microcontrast being so high that edges are more sharply defined. In essence, doing in the glass what software typically does to sharpen.