It doesn't necessarily mean the lens you shoot with the most, the one which covers the most useful range, or the sharpest one you have. Which lens gives you the tingles every time you put it on your camera? If you were being mugged, which one would you be least willing to sacrifice? Which one has the greatest story to tell?
Mine is my 'Killer Whale', the 70-200mm f/4L IS. I was in the process of getting a 24-70mm f/2.8L, when the sales rep at the camera store disclosed to me that they had a second hand 70-200 f/4L IS out the back. I had been lusting after this lens for quite some time, but I figured the 24-70 would give me a more useful range, so I was looking to get the Brick first. When they told me of the 70-200 sitting out the back going for AU$900, I went home, pulled the money out of my savings account and picked it up the next day. The shop was selling the 70-200 f/4L IS new for AU$1500, and Canon were about to bump the price up to about $2000, so I made a killer of a saving.
After a couple of days, I noticed it wasn't focusing back from infinity when I pointed it up, and it had to hunt quite a bit when horizontal. I took it to Canon repairs, and 3 weeks and $300 later, I had the lens back in my hands (and on the camera).
Even though I find the 24-70 f/2.8L to be slightly sharper, and the f/2.8, the MFD and the focal range to be more useful, it still doesn't have the same 'feel' as the 70-200. I can't pinpoint what it is, but the Killer Whale just feels brilliant.
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