DxOMark has just published their guide to the best prime lenses for the Sony α7R II. On the DxOMark blog Paul Carrol writes, " The camera/sensor is only half the equation, however, and to maximize image quality, the Sony α7R II’s high-resolution 42.4Mp sensor demands — and frankly deserves — top-quality glass to realize its potential." 

The writeup continues, "We’ve now tested a total of 17 FE mount lenses on the α7R II, including primes and zooms, covering focal lengths from wide-angle 21mm through to super-telephoto 300mm. These include some of the best-performing and sharpest lenses we’ve ever tested, with overall DxOMark scores ranging from a superb high of 49 points to the lowest, but still respectable, 26 points. Our in-depth analysis of the best lenses for the Sony α7R II rounds up results for many of these lenses, to give you all the info you need to make the right buying decisions. To make it simple, we’ve categorized these lenses into five groups: standard zoom, telephoto zoom, and primes for portraiture, landscape, and general use."

Their overall top pick is the 85mm f/1.4 G Master followed closely by the 55mm f/1.8 Sonnar T* and the 90mm f/2.8 Macro G OSS.  

For portrait photography, the 85mm G Master takes the top spot. That lens on the α7R II has the third highest rank in the DxOMark database. "Portrait lenses offer a short telephoto focal length that compresses perspective and keeps facial features in proportion, while also featuring fast maximum apertures to help throw the background out of focus and isolate the main subject. There are three portrait prime lenses in the 85/90mm range available for the Sony α7R II, including two of the best-performing lenses we’ve ever tested. Topping the group is the Sony FE 85mm f/1.4 GM, which, tested on the α7R II, is the third highest-ranked lens/camera combo in our database, with an overall DxOMark score of 49 points."

See the full scoreboard at DxOMark.com