I tested almost all and I bought/sold them in this order :
Canon 17-40 : best price/quality if you really want Canon, as sharp as 16-35 II
Canon 16-35 I : f2.8 but softer lens
Canon 16-35 II : overall good, but way overpriced for what it is (it's still not perfect for this price!!!)
Canon 17-40 : yes I returned a second time to this one, you can't miss
Tokina 16-28 2.8 = The Nikon 14-24 for Canon but WITH AF, also corner to corner sharpness and only a bit more expensive than the 17-40!!!
Conclusion : Tokina is the best UWA that I found, only negative is sometimes flare (bulb design like the Nikon and no filters like the Nikon), it's like a perfect copy of the Nikon, it's that good!!!!
When I need filters, I just put them on another lens like my 24-70 2.8 



