All lenses mentioned are good options from what I`ve heard. I bought the sigma 8-16 last week, and it rocks! Very very wide, and amazingly sharp. The reason I bought this instead of the alternatives, is that it is the widest. If you are gonna be a bear, be a grizzly!
The only downside with the 8-16mm is that it does not take filters, and it is APS-C only.
+1 I love my sigma 8-16 + 7D combo especially for landscape pix and vids. Super wide. I actually use it on my FF 5dmarkiii also but only at 16mm anything less and you'll get really bad vinetting.
Another +1 for the Sigma 8-16.