As any decent photographer knows, there are a lot of factors that go into making a good lens: handling, color rendition, bokeh, focus speed, flare, build quality, and so on. One significant factor is sharpness, and it is significant for two reasons.
• Sharpness can help you make sharp photos.
• Sharpness is one of the most over-emphasized qualities of a lens.
I cite the second reason because lens sharpness is easy to turn into a goal unto itself, which obscures the real goal of our craft, making images. It can also be somewhat misleading since some of the world’s "crappiest" lenses are sharp in certain circumstances.
With that said, I will add that I have owned some amazing lenses over the decades, many quite sharp, and sharp when I needed them to be, like wide open in low light at marginal shutter speeds.
In my pantheon of imaging, though, the lens with the actual, absolute “wow this is amazing” sharpness in my bag is the 60mm f/2.8 AF Micro-Nikkor.
I didn’t buy it on purpose. It came with a camera I bought on eBay a couple of years ago.
This thing continues to dazzle me every time I pick it up, even though that’s not all that often since it’s not a great focal length. A better focal length for macro photography is 100mm or longer, mostly because focal lengths shorter than that tend to place the photographer so close to the subject the lens actually casts a shadow on the subject.
I sometimes have to search for a reason to use the 60mm, but when I do, yikes, this thing is good.
Another bonus characteristic of this lens is the straight nine-bladed aperture which creates dazzling 18-point sunstars around bright points of light, which I love.
If you happen upon one at a bargain price, I recommend it.
Read more on Richard's photography teaching blog: http://richardbarron.net/cameras/