Canon EF 100-400mm f/4.5-5.6 IS USM Lens

Let me first say that you will not obtain as good quality images with any zoom lens compared to fixed focal length (prime lens)
Saying that this lens is as about as good as they come for a medium to long zoom. The build quality is excellent as you would expect from an 'L'series.
It is reasonably fast at f/4.5-5.6 and the motor is very quiet. The IS is a big bonus with long lenes like this, helping you to shoot at lower shutter speeds without the images being ruined by camera shake.
Images are very sharp at about f8. This lens is a true wildlife and sports lens and performs well in this field. You can of course add a teleconvertor for even longer focal lengths however you lose 'F' stops and quality and you should really think of splashing out on a longer prime lens if, for instance you want to shoot small birds at distance. Or of course try ways of getting closer to your subject, ie a bird hide, camo screen.
I strongly recommend this lens.