I'm a Canon fanboy, but here are some general things to think about.
You want a bit of focal length, too close and the subjects can start to show distortion. 50mm on a crop lens is generally accepted as fine, sometimes as short as 35mm. Probably wouldn't go shorter than that unless you're trying for a specific look.
Faster lenses give you more flexibility, but it's not hugely important since you'll probably be using a tripod most of the time. A fast lens also gives you the option of narrow focus planes, which is sometimes a greatly desired effect.
Look for a tack sharp lens, at varying F-stops, from the center to the edge.
Minimal distortion (CA, vignette, etc) is highly desirable.
Image stabilization is not needed (tripod).
Tilt-shift lenses are generally considered king of the hill for product photography. Canon 45mm f/2.8 TS-E for crop bodies and Canon 90mm f/2.8 TS-E for full frame are common.
Consider a macro lens if you want to do extreme closeups.
