The Canon ƒ/1.2 is a bit of a specialist, and needs care to get the best from it, since it suffers from focus shifts as you stop down. Remember that all AF is done at maximum aperture.
The Canon ƒ/1.4 is not bad optically, but it's not stunning either. The problem is that it suffers from a really bad old fashioned Ring USM motor, which is neither fast not quite. You also don't get FTM with that type of USM motor. The worst though is that the focus mechanism is really delicate, even just a gentle knock to the front of the lens can kill the whole focus, not just the AF. Ideally you keep the lens hood fitted correctly at all times, even in the bag, to help prevent damage occurring.
The Canon ƒ/1.8 is the cheapest of them, and although optically they are all pretty much identical, Double Gauss lenses at what is about the normal for the format is what you are going to get. The first of them was quite nicely made, although AF wasn't great. Metal lens mount on the Mk I. The Mk II is much less well built, and the AF is still not great. Plastic lens mount on this one. The latest is the STM, this has a better build quality, and by accounts the AF is quite a bit better.
If I were looking for a 50mm lens, and wasn't looking to spend the minimum then I would be looking for a lens that wasn't made by Canon. The best with AF is by all accounts the Sigma 50mm ƒ/1.4 Art, and some rank it alongside the Zeiss Otus 55mm ƒ/1.4, although the Otus is MF only. The lens that probably is most worth looking at for mid range is the Tamron 45mm ƒ/1.8 SP Di VC USD, which IIRC is close to the Canon 1.4 in price.
It is important to ask yourself what camera format the recommendation for the 50mm is being made for. On a camera with a 35mm format sensor the 50mm has been sold as the standard lens for many years. 50mm is just a little longer than the sensor normal of 43.3mm, so it gives a very natural looking image. it makes the perfect walk around prime lens. On an APS-C format sensor though 50mm is a short telephoto, even if the lens design isn't. So on an APS-C sensor body the 50mm is the perfect introductory prime lens for portrait work, filling the role of the 85mm on the 35mm format. If you are looking for a normal focal length for APS-C then you should be looking in the 28-35mm focal length range, which is from almost perfectly on the normal, to just a nice length above. A 35mm is equivalent to those systems that sold 35mm cameras with a 56mm lens. The various Sigma 30mm ƒ/1.4s would fill this role well on APS-C.