I have owned the 50 f/1.8, the 50 L, and the sigma 50 f/1.4.
The best is the 50L hands down. If you own 2-3 lenses, and if you shoot a lot in the 50mm focal length, this should be one of those lenses. However, the sigma 50 is a great bargain, especially second hand, in my opinion. Just make sure to shoot a load of test shots on your own camera with it beforehand to check for back/front focus issues. I chose the sigma over the canon f/1.4 because it is smoother in it's rendering, especially for out of focus areas or bokeh. I don't have any experience with manual only focusing lenses for a Canon dslr.
May I ask why is the 50L the best hands down? I have a sigma but still wondering what quality makes the 50L better?