Choosing a Prime lens over a zoom will give you the advantage of a wide maximum aperture that will enable you to shoot images in low ambient light and/or images with very shallow depth of field. They typically offer superior sharpness to a zoom lens.
Two of the best Full-Frame E-Mount Primes for the Sony A7 cameras (identified by their ‘FE’ prefix) are the Sonnar T* FE 55mm f/1.8 ZA Lens and the FE 90mm F2.8 Macro G OSS
They have been tested by DxO as two of the sharpest autofocus lenses ever made (by any manufacturer) and are highest scoring E-Mount Primes (even outperforming the Zeiss Batis and Loxia primes). On the A7RII the 55 can be used in APS-C mode and acquire an effective focal length of 82.5mm. Although the Megapixel count will drop to 18 in APS-C mode, this is still enough resolution for most photographers. 4K examples of images captured by the FE55 can be viewed here: https://flic.kr/s/aHsk9h7RVS
If you do not want to shoot in crop mode and Sony’s FE85F14GM lens (the ‘Bokeh King’) is beyond your budget, the exceptionally sharp 90mm Macro can be used as an excellent portrait lens. 4K examples of images captured by the FE90 can be viewed here: https://flic.kr/s/aHsk9oYDMP
For those looking for something a little wider Sony offer a choice of two different 35mm Lenses. The FE35F14ZA has a wide f/1.4 aperture and makes an ideal Cine lens with its clickless aperture option and smooth focussing capability. The FE35F28, on the other hand, is Sony’s smallest FE prime and makes for the perfect ‘Street Photography’ lens. 4K examples of images captured by the FE35F28ZA can be viewed here: https://flic.kr/s/aHska3AYH7
B&H have currently discounting the FE55 and when bundled with a Sony Battery grip the saving increases again. Go to this link to take advantage of this offer: Sony SEL55F18ZA
A full run down of the equipment I personally use can be accessed from the ‘What’s in Mark’s Bag’ link: http://www.markgaler.com/whats-marks-bag