Conclusion

• All candidates produce a very smooth bokeh and good to excellent center sharpness already at open aperture.
• The Leica Summilux suffers a bit in the midframe and border areas due to higher astigmatism compared to the other candidates. It also shows the strongest field curvature - at least when used on the Sony A7R. You can see the impact when you compare the photo of the fisherman with that from the Otus and the similar view with the Sigma. The Summilux produces a very nice background blur around the separated fisherman but at the borders the background gets sharp again. In fact, it is no "edge smearing" as ofthen stated, instead it is the result of field curvature as demonstrated with the test charts and the edge focused crops as well.
  
• The Sigma Art 50/14 delivers a very good center resolution already at open aperture as well and sigificantly better midframe (half way between center end edges) and edge results than the Summilux, it also shows less field curvature when used on the 36 MP sensor of the Sony A7R. But the quality is not as consistent across the whole frame (especially in the edges) as the results shown by the Zeiss FE and the Zeiss Otus.
  
• The Zeiss FE 1.8/55 delivers a surprisingly good result already at open aperture accross the whole image frame and shows only little field curvature. The only disadvantage of the Zeiss FE 1.8/55 is the missing 2/3 aperture compared to the f/1.4 lenses. Already at f/2.0 the Zeiss FE 1.8/55 begins to produce moiré in the reproduction of the rastered print what can not be accused to the lens. It is an indication that already the resolution of the AA-filterless 36 MP sensor is reached here.
  
• Both - the Zeiss FE and the Sigma Art - show some minor, but easily correctible CA. The Sigma produces nicer "bokeh balls" from highlights in the out-of-focus area.
  
• On the 36 MP sensor of the Sony A7R the Zeiss Otus outperforms all other lenses in a significant degree. Even at open aperture it already reaches an edge resolution that the other lenses can not deliver before stopped down to f/4.0. Field curvature is negligible and edge resolution almost reaches the sensors resolution already at F2.0. At f/2.8 resolution and contrast are on a level that practically can not be raised anymore when attached to a 36 MP sensor. It seems that this lens is already designed to challenge sensors with 100+ MP.

Summarized I would say that the Zeiss FE 1.8/55 still delivers best value for its price when used on mirrorless full frame E-mount cameras like the Sony A7 / A7R in a very light and fairly compact package. Sometimes you may miss the 2/3 additional aperture but it shows extremely good performance across the whole frame and makes additional points with its AF capability (but the others can be manually focused with better feedback). For me it is stil the "Otus light".

As the Sigma Art 50/1.4 DG HSM is primarily designed to be used on DSLRs from Canon, Nikon and Sony, here it will be a very attractive choice outperforming the previously available selection from the camera manufacturers.

The Otus still marks the reference for me. With its incredible detail, contrast and smooth bokeh across the whole image frame, it is a lens without any optical compromises but you will think twice before putting it into you photo bag due to its weight, size, price tag and lack of autofocus.

Finale

Some more samples from the Sigma at open aperture, just for pleasure: