Well, I guess everybody has a nemesis and mine is the Canon EF 24-70mm f/2.8 USM L. It took me 4 (f-o-u-r) samples of the lens to get a good one - please note: "good", not a "great" sample. The first three variants showed rather hefty centering defects which spoiled the results quite a bit.
Source:

Canon EF 24-70mm f/2.8 USM L - Photozone Test Report / Review

Qui est quand même un site pas trop mal