Canon's legendary 1200mm f/5.6 L, which they claim is the world’s largest interchangeable SLR AF lens, in terms of both focal length and maximum aperture.  It is compatible with the Extender EF 1.4x and 2x, which turn it into a super-super-telephoto of 1700mm f/8 or 2400mm f/11.

Unfortunately we don't have one of these, yet.  We recently saw one advertised for sale at US$99,000 (plus $US935 postage to the UK!) - which is surprising enough in itself, as there are supposedly only about 10 or 12 in existence - but we don't have that kind of loose change lying around.  Maybe next year.

If you think we should be keeping our eyes peeled for one of these coming available - and looking very carefully under the sofa cushions to see if we can scrape up the money - then do please drop us a line!