The best 50s for bokeh will be the f1,4s out there (starting with the Takumars up to the FA) according to many. I have owned the A-50 f1,4 and the F-50 f1,7 and I will agree that the f,4 is marginally, very marginally better. There's a creamy effect that's not quite the same with the f1,7. But the latter is excellent in its own right, especially at wide apertures. The bokeh is creamy enough, even with busy backgrounds.
One type of lens, more expensive, but delivering even better results, is the 50 macro line. I own the FA 50 f2,8 macro and even though it's ludicrously sharp, the background is amazingly smooth. It's my best lens regarding bokeh. It's also normally quite expensive, except when you find a deal like I did