All of them, they have different target audience. In my opinion the 1.8 is a nice cheap lens that can teach you a lot about photography if you are a newcomer. 1.4 is very nice and sharp upgrade to the 1.8, the 1.2 is an amazingly sharp lens, one of the few to reach such speed but in a real world it is not so useful since very small dof @TA.
With the 1.2 one can do more wrong than right. The 1.8 is the other way round. The 1.4 is somewhere in the middle. The 1.8 is solid performer for many applications. The other lenses are special interest lenses. You can see it, when you compare landscape pics taken with these lenses. The 1.8 is great to catch a moment. The 1.2 requires some preparation.
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