The "best" zoom lens is defined by the need of your shot.
Labeling something the "best" of a broad category is difficult, and zoom lenses are a broad category. Depending on what you are needing for your shot, the best zoom lens will vary. Photography is the art of writing with light (literally) and there is no "one way" to shoot anything. There are, however ideal zoom lenses to use for each situation, though in the end it all depends on the relationship between lens and photographer. That is, trial and error.
Nikon AF 70-180mm f/4.5-5.6D Micro
The Nikon 70-180mm AF Micro is the only full zoom micro (meaning macro, that is focusing on very small detailed objects such as a bug or flower) lens so far produced by Nikon (or any lens manufacturer for that matter). The focusing length distance is a mere 37cm (that means you have to be at least 37 cm away from an object to photograph it). The lens was manufactured from 1997 to 2004, and has been discontinued. Nikon has yet to create an equal or greater equivalent. You can find used models for sale anywhere used photo equipment is sold, but it was only after its discontinuation that the majority realize its true value, and thus its used price is severely higher than its original "new" value.
Nikon 28-200mm G
The Nikon 28-200mm G lenses are the "best" zoom lenses in terms of value and price. Why cheap? They are made out of plastic and were first produced for film cameras at the close of the film era (early 2000s). The advantages of the 28-200mm G is its sharpness, even at zoom, even sharper than many of the new (more expensive) high-tech lenses. The lens is light (plastic) and easy to carry and whip out at a moment's need. Many camera reviews cite it for its ability to focus at every length better than most telephoto lenses (Ken Rockwell) and has an auto focus faster than even the newest AF lenses.
Nikon 135mm f/2 DC AF
Another "best" in zoom lenses, the Nikon 135mm defocusing (DF) lens is the best zoom lens for creating stand-out portrait shots against soft or blurred backgrounds or foregrounds. Defocusing the background into a hazy cloud almost representative of a dream sequence is a feature most photographers lust after. The lens was never popular in the U.S. due to the misunderstood term "defocus", which to many doesn't sound like a good idea. The 135mm DC allows for a sharp focus and a sizable zoom and is the best hand held (no tripod necessary) lens for portrait photography.
Nikkor 70-200mm f/2.8 VR
The AD-S Nikkor 70-200mm is revered by many photographers as one of Nikon's best (and newer) telephoto zoom lenses. The lens has unparalleled vibration reduction built in, a 4.9-foot focusing distance (at automatic) and a 4.6-inch focusing distance at manual focus. The lens is compatible with virtually all Nikon teleconverters, and the focal speed is unparalleled among Nikkor lenses. This lens is ideal for sports and sideline shooting of fast motion. It is among Nikon's most expensive lens models, beginning at about $1,700.
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