That is hard to say because each lens has a different purpose in life. Those that couldn't perform are gone.
My Canon EF 70-300 nonL is light weight and tack sharp. I can take a hand held picture of a small street sign 3/4 mile away, blow it up around 200% and have a legible image. To me that is simply amazing. It also provides good blur for normal pictures. It has a good IS and focuses reasonably fast.
My Canon EF-S 10-22mm is also tack sharp. It provides me a close up perspective and is outstanding for indoors, landscape and scenic photography. Again focusing is reasonably fast.
My EF-S 18-135mm is tack sharp as well. It is my general purpose workhorse. It also has IS and is quick to focus.
So I guess I will just have to nominate all three.