I see lots of posts re: "what should I buy" but I've never seen any on what people have got rid of, and more importantly why.
Here is what I've sold in the past and why (in order).
Canon 18-135 - was never really impressed with the IQ. Always left me wanting more.
sigma 24-70 2.8 (v2) - AF wildly inconsistent. Bokeh very unattractive (I would call it 'crunchy'). I never really trusted this lens, or liked the photos it took. Sold it for canon 17-40 on a whim, as I needed a gen zoom quickly for a holiday.
canon 17-40 - The IQ, flare control, contrast and AF were all great, however I was never excited by the zoom range on crop, so sold it for 17-55. However I regretted selling it once I bought my 5d3.
canon 2x TC ii - I bought it when it was mis-priced in a local camera shop. However, the IQ was crap so I never used it.
Canon 7d - was a great camera, but after 3 years of (heavy) shooting it started to show signs of wear so I decided to go full frame and replaced it with a 5d3.
Canon 85 1.8 - sold for 85 1.2 ii. However wish I kep the 1.8 aswell as sometimes the f1.2 is just to big/heavy/slow AF to use out and about (althought when it nails it the results are glorious).
canon 50 1.4 - sold for 50L 1.2. Best decision I ever made. Wish I did it 3 years ago. It now lives on my camera.
canon 100mm macro - a lovely lens. However I took it to a humid butterfly farm which killed the aperture blades (they got stuck). I got it fixed by an authorised canon repair centre but it was never the same lens after I got it back. It still worked fine, but the AF was so slow after. So sold it, on the cheap
, and replaced it with the 100L
canon 17-55 - just sold it as I upgraded to full frame and no longer really use my 60d (I keep my 60d for backup purposes and when I go to the zoo). Plus to be honest, I've never really been impressed by wide open IQ or contrast. Given my shooting style (run and gun) I don't really have the need for IS and I prefer shooting at wider apertures than f2.8.
sigma 10-20 - A wonderful lens. Never had any complaints, however I no longer use it because I upgraded to full frame. so gonna purchase a FF equivelent like the sigma 12-24 or canon 16-35.