Summer 2013 - With my recent venture into Medium Format it is a bit too soon to pick a favourite as I only currently have two lenses and love them both - both by Carl Zeiss the 80mm f2.8 and the 135mm f5.6 Makro Planar. The images quality from both is superb.
In view of the change in the way I am working, I sold the x2 Extender and am reconsidering keeping the Canon 16-35L lens as it just does not get the use it did although I still like the lens.
Summer 2012 - I finally invested in a Canon 100L Macro lens, primarily due to the number of commissions for fine art reproductions I was getting. It has also been a wonderful lens with which to enjoy close-ups of flowers and I now realise that whilst my earlier alternatives were fun and did realise some good images, this lens is really in a class of its own.
I sold the 70-200L zoom lens as I did not use it that often, preferring the 200 prime as it is significantly lighter and better balanced with the 5DII and also has better image quality.
All photographers have a favourite lens - mine has long been my Canon 200L f2.8 and is the 35mm lens I use most often due to it's sharpness, colour rendition and extreme portability. Being fast and relatively light, it is great for handheld shots, especially in low light Acquired many years ago to replace the 75-300mm, which whilst a nice size, just isn't in the same league.
Another favourite is the amazing Canon EF 300mm F4L USM prime lens, acquired to replace a Sigma 170 mm - 500 mm - f/5.0-6.3 DG that I found too big, too heavy and unwieldy, the images rather lacking the contrast and dynamic range which the Canon L lenses give. The quality produced by this Canon lens is legendary.
A Canon EF 50mm f/1.8 II Lens handles the standard range and is perfect for very low light or where compactness is needed. This little lens has got to be the best value of all - trouble is, I always forget it's there but when I do use it the results are always sharp and well defined. It gives brilliant results on the 5D making a light and compact solution for travelling and is just such a fun lens to use.
My photography improved after acquiring the primes as I started thinking more about my position when framing a shot instead of relying on the zoom. That said, for a while I did use a Canon EF 70-200mm f/2.8L for occasions such as working from a hide or a tripod, when it is more difficult to change focal length quickly or in very dusty or sandy surroundings. Not quite as sharp as the prime, much heavier and I did find it a bit unwieldy at first - so much so I considered replacing it with a Canon 100 mm-400 mm - f/4.5-5.6 L IS on the basis that if I had to carry a heavy zoom it might as well cover a greater distance and Arthur Morris seemed to rate it very highly for bird flight shots. However, I found the bigger lens that bit too heavy and never really got on with the telescopic push-pull zoom, so after a week it went back and I kept the 70-200. It got used more often than I expected as it was especially useful for sports, events and any kind of fast moving activity. I used to shoot some non-league football and other sports events so for that this lens was perfect. When I stopped doing that type of work I sold it on as it was no longer being used.
For wide scenics and short focal lengths, I still use the Canon EF 16-35mm f/2.8L II USM, acquired to replace a Canon 20-35m, which in turn replaced a Cosina 19-35mm. The Cosina was quite good but lacked vibrance and was quite soft around the edges. The Canon 20-35 was better but I never quite got on with the diaphram on the lens and always ended up with wonky horizons. The 16-35L zoom gives excellent results with superb colours and contrast even when used wide open and at wide angles, although some vignetting shows in the corners when used with a full frame camera.
I sold a Sigma EX 105m 2.8 Macro lens, and for a while used a Canon 500D close-up lens on the 200L and 300L with some nice results, making a very lightweight alternative to carrying a separate macro lens and good to keep in a pocket 'just in case'. I have also used it with great success combined with extension tubes on the 300L. The 200L with x2 extender and extension tubes produced some really interesting results and was fun to use but a tripod is essential as the slightest movement spoils the image. The depth of field with this combination is next to nothing but as I tend to use selective focus, I achieved some very pleasing results.
An impulse buy was a Lens Baby 3G complete with tele and macro kit and adapters. It looked like the kind of lens with which to have some fun and I had seen some of Tony Sweet's images taken with it so thought I'd give it a try but I never quite got the hang of the technique despite several attempts. I did manage a couple of interesting shots but as I hardly ever used it I sold it on.