Advice on best lens to buy?

I'm not to educated when it comes to my Nikon D5000. I'm interested in purchasing a new lens though and I'm wondering what's the best one for what I want to achieve. I'm looking for a lens that will give the best picture quality, thats crisp and sharp. The lens would be used for mostly portrait photos. The most important thing I'm looking for though is a clear image thats a perfect quality. Any suggestions?

For portraits, the favorite is a 50mm lens. Depending on your budget the 50mm f/1.4 would be best, but the f/1.8 is also very very good

make sure you get the AF-S (G) version of the lens instead of the AF (D) version.AF-S will autofocus, AF will not

As previously stated, a 50 1.8 AF-S would be great for portraits. For optimal center sharpness, shoot at f3.5 or 4.5.

Either the stated above or the 35mm vr 1.8f

Nikon 50mm f/1.4G AF-S lens produces sharp pictures and great color and contrast. It is also perfect for portrait and other general purposes (semi-macro etc). This lens also produces nice bokeh. The picture quality and bokeh quality are comparable with the other (older) Nikon 50mm lens which are famous for being sharp. Overall, this is a very versatile lens.