The Canon EF 400mm f/2.8L IS II USM is as good as it gets for a super-fast, super-telephoto prime lens.
From its rugged build quality and expansive feature set to its brilliant optical performance, trick focus modes and rock-steady image stabiliser, everything comes together perfectly.
The only downsides are that it's big and heavy to use, and massively expensive to buy.
Superb image quality made all the more effective by excellent autofocus and image stabilisation systems.
It's very big, still heavy despite the Mark II's weight loss, and extremely expensive.
Even with its disadvantages, the Canon EF 400mm f/2.8L IS II USM would be a great super-telephoto lens to hire for special occasions.