For Canon I have the 16-35 2.8 L lens and so in love with it. I also have a Sigma 10-20mm F4-5.6 for when I want something a bit wider but since I bought the Canon lens I do not think I have used the Sigma but I rarely need something that wide and it just a personal preference for me.
EDIT: The Sigma I really liked until I bought the Canon and would recommend it for the optics quality and price.
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