Canon 17-40/L, when used as an UWA on FF gear, not as a standard lens on crop.
Why do you say this? I use the 17-40 as a standard lens on crop and it performs very well for the most part IMO, as I understand it it's biggest weakness is used at the wide end without stopping down on FF. Using it on crop negates most of this issue.
The 17-40 is Canon's best full frame WA zoom.
I'm using a 17-40L on crop as a standard zoom because I plan (for CR regulars yes, really :-p) to upgrade to ff sometime and most of all the L is sturdy and sealed which makes a great difference because I regularly shoot in snow and dusty/dirty outdoor conditions. Just be sure to replace the lens hood with a 83J...
... however @f4 the 17-40L is clearly lacking in sharpness and except for the above reasons I really wouldn't recommend it on crop, even though it has a very good price nowadays esp. with Canon cashback rebates.
Sure, it works as a standard zoom on crop. But used this way I wouldn't call it a "sleeper lens".