Back in the 70s and 80s, when an SRL was purchased, it invariably came with a 50mm lens. Some will choose a 50mm f1.4, as an upgrade to the f1.7/1.8/1.9/2.0 lens that normally would come with the camera. There is a good reason for this. The 50mm lens has a field of view similar to that of the human eye vision. It is much easier to compose and use than say, a 28mm wide angle, not to mention the usually fast aperture, which made it usable even in very dim lights. In other words, the 50mm lens is a very versatile lens. This hold true today.
But which 50mm lens would one get? I don't think anyone can answer that question, because there are literally hundreds of different ones made by so many manufacturers, and they cost anywhere from five to thousands of dollars. For NEX users, the choice is even broader, simply because you can mount more lenses on it than any other interchangeable lens cameras out there.
Even though I can't recommend a specific 50mm lens, if this is the first time you are trying out a manual focus lens on digital camera, I would suggest starting with something cheap. Cheap in price, but not in performance. This allows you to try it and see how you like focusing manually. Really, manual focus lenses is not for everyone.
What manual focus lenses are still cheap in price? My favourites are Canon FD and Minolta SR/MD mount lenses. Often the Canon FD 50mm f1.8 can be had for around $10. I tried a New FD version and it was optically excellent. Optically equivalent to the EF 50mm f1.8 II, if not better. The other good choice is the Minolta Rokkor 50mm f1.7. This lens is often overlooked but optically it compares well with the modern equivalent. Other low price but good chooses are lenses from Konica KR, Olympus OM, and of course the supremely well made Pentax Takumars. If you don't like stuff made in the east, there are low priced Pentacons, Meyer-Optiks, Carl Zeiss you can also try, mostly in M42 mount.
The danger here is that once you tried manual focus lenses, you may get hooked and even addicted. Yours truly is a deeply addicted 50mm ( specifically 40-60mm) lens user. No amount of auto focus lenses treatments has any effect on the condition. Even extreme treatments with the L formulation did not help much. Below is a snapshot of the a collection taken two years ago.
Fabulous Fifties. Click to see larger.