300mm x 3
A quick look at the Bokeh from three 300mm Nikkor lenses.
NIKKOR 300 f/2.8 ED IF AI-s
NIKKOR 300 f/4.5 ED IF AI-s
NIKKOR 70-300 f/4.5/5.6 VR AF

 300mm lenses are just about the minimum focal length needed for capturing spectator events such as sports and concerts. For most animals longer focal lengths are advantageous. At 300mm you get a much closer view than most camera/lens combos you see people using so you'll have the opportunity to create a better final image.

  The faster the lens the better. (with caveats) An f/2.8 lens vs. something slower like the 2 others compared here enables the user to use faster shutter speeds and the wider aperture will separate the subject from it's surroundings to a higher degree, as seen in the images below... making for a less distracting background. BOKEH!

  The drawbacks to faster lenses is they are heavier and bigger than slower lenses of the same focal length and once you get to the 300mm mark they get a LOT heavier and a LOT bigger. This means a tripod or at least a monopod will be required for any timely shooting. They are also MUCH more expensive. Filters alone for the fastest beasts will set you back as much or more than many lenses!!! Are they worth the extra cost? That depends on your shooting needs. If your a weekend shooter then in my opinion you'll be much happier with the smaller 300mm lenses. If your shooting demands the highest quality in the lowest light requirements then you'll want the faster lenses.

  Notice the Bokeh in these 3 images. When we shoot the 300mm f/2.8 shot at f/2.8 the background becomes smooth and edgeless. The 300mm f/4.5 can't compete with the f/2.8's Bokeh but it's still nicely diffused. The 70-300mm f/4.5/5.6 is only able to use f/5.6 when shooting at 300mm and is noticeably less diffused but still provides good separation in the example below. For most shooters the 70-300mm Zoom is a sensible choice because of the zoom, VR (Vibration Reduction), price, size and weight. Of course autofocus will be important for many shooters. Nikon makes several 300mm lenses and all are autofocus nowadays.

As a side note: Prime lenses (those that are not zooms) are almost always better optically at their specific focal length than those lenses that do zoom.