I voted for the Canon 50mm f/1.4 USM (as long as it's true USM - which I expect any replacement to the current 50mm f/1.4 will be).
Having written that (& voted) - I would like to add 2 other suggestions:
1) With last month's announcement of Sigma's new 18-35 f/1.8 DC HSM Art for APS-C, as a ground-breaking fast zoom - I would like to see a manufacturer (preferably Canon) come out with a complementary fast zoom lens, in the 30mm - 70mm range. I realise having a f/1.8 (or f/2) zoom all the way from 30mm to 70mm might be biting off more than one can chew (without compromise in size, image quality, etc). So even having a 35mm - 60mm range could still work well. USM focus to be used (or STM - at a push). IS not a priority for this lens.
2) Canon 50mm f/2 USM IS. I would probably prefer the 50mm f/2 USM IS over a Canon 50mm f/1.4, due to the smaller size / potential lower cost. Obviously if the 50mm f/1.4 is sharper @ f/2 than a 50mm f/2 @ f/2 (wide open) - I would take that into account. The recently announced Canon 35mm f/2 USM IS looks like a great lens, awesome IQ wide open: sharp, contrasty, great bokeh, etc. So I see a Canon 50mm f/2 USM IS as a logical near future release.
I would be keen to hear other's thoughts / input / calculations (eg of weight, size, filters, estimated cost, etc) on the above lenses.