Personally I've got the typical 1.8 because of the price (used for about 70€).
Well... on the day my 5D Mark III came, my 50mm 1.8 II died. The front element fell off as a plastic part was broken and fell out on the floor and I couldn't find it anymore in order to find a way to repair the lens. Damned. OK... the 50mm 1.8 was a nice little lens with a great image quality but it's build quality... well... you just heard the story.
My 5 lens kit was... Sigma 12-24 (most extreme wide angle - yay!), 24-105mm (nice everyday lens), 100-400mm (nice tele lens), a 100mm 2.8L Macro for portraits and macro shots and a 50mm 1.8 for low light, product photography and portraits. I was pretty happy with my set of lenses but now I'm sitting here without any 50mm lens, which is... well... sad.
Just checking the lenses around the 50mm range:
40mm 2.8 STM - pretty nice, sharp and small but shorter and "only" 2.8, not too expensive. It would look nice on the 5D3 with a battery grip attached
50mm 1.8 - like the 50mm 1.8 II but with a better build quality
50mm 1.8 II - the one I had and just died, center sharpness was good
50mm 2.5 Macro - well... I have a macro lens and 2.5 isn't really a bright lens but used it's not much more expensive than a 50mm 1.8 and it seems to have a pretty good sharpness and build quality and 2.5 is still OK - maybe that's the lens I'm looking for? I just didn't hear so much about it.
50mm 1.4 USM - quite sharp, quite fast, payable but the AF seems to be it's weak point
50mm 1.2L USM - pretty expensive, weather sealed, nice bokeh but not really sharp
50mm 1.0L USM - hardly to get anywhere and pretty damn expensive and not sharp
Sigma 50mm 1.4 EX DG HSM - nice bokeh, good sharpness but many people seem to have focusing problems with it. But there are other people who are talking about focusing problems with the 50mm 1.8 II and I had none.
I even thought of a 85mm 1.8 USM for its image and build quality, its speed and as it's still payable. AND it's about the angle I'm used from my 50mm 1.8 II on my crop frame camera. But then... 85mm are pretty close to my 100mm lens...
Any more compatible AF lenses I just forgot?
OK, let's get the list a bit shorter... although I really liked my 50mm 1.8 II I'm not going to get another 50mm 1.8 II. I'm not always so nice to my equipment but it should at least survive. A small plastic part killing the lens is not good. I'm not going to get the 50mm 1.0 - just not sharp and much too expensive. I'll leave those to the collectors. The 50mm 1.2 is pretty expensive as well and not as sharp as the 50mm 1.4 (although the 1.2 has a nicer bokeh) but for that amount of money it just doesn't feel "enough" if I can get a cheaper lens with only few disadvantages but also advantages over the much more expensive one.
So I guess the short list:
50mm 2.5 Macro
50mm 1.4 USM
Sigma 50mm 1.4 EX DG HSM
aaaand... last but not least the cheap, sharp and very small 40mm 2.8.
Usually I find a lens, think "Wow!" or "Naaa..." and decide whether I need it or not. But this time I think I need some advice, some talk about the lenses. I have to admit: I'm especially kinda interested in the 50mm 2.5. Has anyone here ever tested it? But then the 50mm 1.4 is close to two stops faster!