As I've written numerous times on CR before, I would be happy with a new Canon 50mm prime.
My main requirements are:
- at least f/2 (up to f/1.4 in an ideal world)
- good IQ wide open (eg sharpn, good contrast & pleasing bokeh, low CA)
- true FTM USM AF (fast & accurate), or STM if not USM
- 4 stop IS
- close MFD
- 58mm filter thread size
- lightweight / small-ish
- metal mount
- less than $800
I like Canon's latest 35mm f/2 USM IS - it meets pretty much all the criteria I have for a 50mm; except it's a 35mm (I've analysed my photos, and I would much prefer a 50mm prime than a 35mm prime). I find I can use one of my zoom lenses at 35mm (eg my Canon 15-85mm). If they could make a 50mm similar, I'd be very happy with that to round out my lens arsenal.
Compare your spec list to the new Zeiss 55mm...it's quite amusing really and puts us all into a very distinct bracket in the market place.