Canon 70-200mm F2.8 L IS

This is a great fast lens. I use it for a lot of photographs. It is great for getting candid portraits or some sporting events (providing you can get close enough to the action).

The image stabilisation allows hand holding in even quite low light conditions if you use the wider aperture settings. Combined with the 5D's high ISO abilities it is a great lens.

It is however very heavy (testing the limits of my tripod I think). Carrying it on the camera is quite an exercise. It also takes a huge ammount of space in the kit bag.