Upload & Sell: On
Your 50/1.8 ... is that version II or I
Also, @ what aperture. It seems pretty soft till you get stopped down past 4.0, looking at this:
As to the other lenses ... are you shooting these hand held, tripod mounted, ambient lighting w/ slow shutter speeds or fast shutter and electronic flash, manual or ETTL, remote release, etc.
Several different factors come into play for image sharpness. You could have a situation where you are front/back focusing also in conjunction with soft glass or other issues. Have you used the DPP software to see where your camera was using for a focal point and compare that with your image sharpness as to whether.
I'd suggest you should conduct some testing to compare your intended focal point vs. your image to see if you are front/back focusing. That and a full range of each aperture for each lens as a baseline.
I'm not a 50's kinda shooter, but the TDP images above wouldn't give me much confidence in that 50/1.8 II ... sample variation maybe, but ...
slrgear.com blur index looks to show some decentering and photozone.de reports some decentering also. photozone.de also points out that this is Canon's cheapest lens. So, given the indication from TDP, slrgear.com and photozone.de ... I think you may be expecting too much from the 50/1.8 II.
Technique, camera and other issues may be a factor, but I think the biggest issue with your 50/1.8 is probably your 50/1.8 ... it just looks to be a cheap soft lens, that doesn't get adequately sharp until you hit around 5.6-11 ... and may be de-centered in addition to that, which could be part of your "hit or miss" results problems too.