Sony 50mm F1.4
Like any fast prime it's not perfect (shooting through all that glass at large apertures inevitably introduces more aberrations), but it has low light capabilities unmatched by any zoom, and image quality unrivalled by all but the most expensive. It's also small, light and unobtrusive, and therefore perfect for discreet shooting. These qualities add up to give a lens worthy of all Alpha users' consideration.
Those in search of a small, relatively sharp portrait or low light lens on their Sony DSLR -- a good match for full frame or APS-C.
Full frame shooters who are highly critical about corner-to-corner sharpness