Canon Zoom Telephoto EF 100-400mm f/4.5-5.6L IS (Image Stabilizer) USM Autofocus Lens

This lens has perfect image quality, seemless zoom and focusing controls, and the optimum telephoto range for outdoor and sports photography.

Clarity is continuous from 100mm all the way to 400mm. With my old 100-300mm lense, I rarely shot over 250mm because I began to lose clarity. I compensated by cropping when I got home. With this lense, 95% of my wildlife photos don't need cropping (not to mention the improved color and clarity). It does work well with my 2x extender, but I find that I don't need it very often any more.

I don't think you could find a better lense for photographing wildlife and sports.

Drawbacks:
1. You pay for what you get. Even though clarity, focus, and zoom are the best available, I wouldn't say they are 10x better than an off-brand lense that you can get for 1/10th the price.
2. Weight. Did I mention wildlife photography? This lense rarely goes hiking. Though when it does, it is worth a few extra aches and pains in your shoulders, back, hips, thighs, quads, knees, calves, ankles, and feet. You WILL need to add the weight of a tripod to your pack as well. You won't be holding this lense up waiting for an animal to strike the perfect pose for long without some support.