Which Canon lens has inflicted most pain on you ?
Repeated duds ?
Hard to master ?
Paid too much for it ?
Got stuck with a defective copy ?
Gave you shoulder/arm aches ?
Etc.
In order to make the thread more lively, might as well enclose an image or two taken with your "lens from hell".
My most vicious lens was the EF 75-300mm f/4-5.6 USM III. Tried really hard to make it work but the lens just wouldn't budge. Finally I accepted the idea it was too much of a lens for me and sold the darn thing.
I can not think of one image that I took with it which I liked, with a possible exception of a little boy torturing a goose. I only wish I had my 300 f/4 on the camera at that time.
24-70L, 3 copies, 5 years, endless focus fustration. Now with a 24-105L and the first wedding shot with it was a resounding success, not one single focus or lens alignment issue. I've got my primes for when I need fast and maximum resolution anyway. It's a shame, the 24-70 is a much better lens, much! It's just not reliable for me. Before some uninformed wannabee starts shouting technique - both my 5D's are perfect at f1.8/2 with my primes, all my other L zooms, etc and I've not had any problems with said L zooms (inc numerous 70-200;s) and primes ever!
I have a love hate relation with my 24-70L. I hate it's bulk and weight so much that I sold it awhile ago. Of course, I re-purchased it a few weeks later because the results from that lens are solid and necessary for event coverage (ie weddings). Recently I bought another 24-105L (sold the first in favor for the old 24-70) because I still hate lugging the 24-70L around for "fun." In the end, I wasted too much money buying, selling, and re-buying, that i should of just held onto my originals.
I had the first version of the 75-300 IS. The IS wasn't as effective as later versions, image quality was mediocre, AF was slow, MF required switching off AF, build quality was lowest consumer grade. All in all it was a frustration to use. No physical pain, but plenty of mental suffering. I traded it toward a 70-200/2.8 IS. Have never been disappointed with the L, and really didn't miss the longer focal lengths until later. Solved that first by getting the 1.4x and then adding the 100-400L.
Well, to a certain degree my first 16-35mm MkII would qualify but I didn't discover that after I sold it and got a second copy a few months later to compare it with....
My 17-55 EFS was the only lens I can say I was regularly disappointed with. I bought it with high expectations, which were never realized. The last straw was when it started collecting dust inside after only 6 months. I sent it in to Canon and got it cleaned, and sold it within days after getting it back. I replaced it with a 24-70, and couldn't be happier.
I went for a 2 hour walk with a 400/2.8 L few weeks ago and it is HEAVY. I'm used to my 400/5.6 L and occational use of 500/4 L but the 400/2.8 L is beyond the line of handholdability, at least for me.
Nothing else imposes more pain on my shoulder, my waist, and my legs than the notorious 600/4L IS But all that is gone when I sit in front of computer and look at the images from it.
Not a canon but mine was a Sigma 100-300/4 . At the time I just didn't understand it. much bigger and heavier than I was used to. wasn't till I sold it I realised how good it was. If I could I would buy another, and why sigma haven't added OS to that isbeyond me as its one of their best lenses.