I keep my 35mm 1.8G on while not in use, because it's usually the first lens I will use if I pick up the camera. Also, mostly because it's the most compact lens I have.
Though, I'll stand corrected a bit - if I want a quick macro dabble using my old extension tube, then 50/2 and the ring pop right off. Read somewhere that those 50s had a relatively flat look in the images thus macro use was actually not too shabby.