If you just buy one.what is the best all around lens for a nikon d80?

Nikon Lens #1 - The Nikkor 50mm f/1.8D AF lens. For $100 or less, there is no other lens out there that can give you sharper images for the money. You can buy $400 lenses which give marginally sharper pictures, but the 50mm f/1.8 lens has the highest sharpness / dollar-spent ratio in the industry.

Its maximum 1.8 aperture also allows you to take great-looking portraits with a blurred background, the kind you see in some professional shots. It's small, it's light, it's great for taking photos in dimly-lit locations where flash is not allowed. Nikon Lens #2 - The Nikkor 18-70mm f/3.5-4.5G ED-IF AF-S DX Zoom lens.

That was a mouthful, wasn't it? Yes, this is a zoom lens, unlike the 50mm f/1.8 mentioned above. Meaning you can take big-group shots in tight locations and include everybody into the picture, and it also allows you to go to 70mm for relatively tight portrait shots.

Hunt around for $320-ish prices, and not a penny more, at your camera stores. Before the availability of Lens #3 that I'm about to mention, this glass was a firm favourite among many casual photographers who shoot travel and vacation images. Nikon Lens #3 - The Nikkor 18-200mm F/3.5-5.6g IF-ED AF-S DX VR.

This lens is so hot. Well, it's hot enough to be out-of-stock at virtually all stores, but you can find one if you're a walk-in customer, and Nikon is bending backwards to ramp up production to meet demand. Why is it hot?

Well, because it's the only lens on the market offering such a huge zoom range, with fast autofocus action and even a VR mechanism that helps you get steady shots without the use of flash. You'll appreciate how useful the VR is if you're in a museum that doesn't allow you to use flash and tripods - the new version of the VR technology Nikon made available on this lens allows you to eliminate handshake at ever slower shutter speeds which beats out the VR capability in more expensive, but older lenses. $750 is all you need to own this lens which you can take 90% of your shots with.