Any modern lens will be very sharp, even the cheapies.
It looks like you already have a 28-200 zoom. I'd recommend a fast (f/2 or faster) fixed focal length lens for general low light use. I'd say 50mm for 35mm film, something else might be better for digital with the mag factor.
Edit: just reread you latest post, the 50mm might work well as a portrait lens on a digital camera.