For me, it was the EF-S 55-250. It always felt 'cheap' on the build, although the IQ was quite acceptable. I found I just didn't have any use for anything over about 150mm. So, it's gone.
My longest lens now is an EF 80-200 f2.8L. Works great, though I rarely go over 150 or so, and that's on my crop 60D!