CHG_CANON wrote:
The single best option is the EF 16-35 f/4L IS. You can go faster and heavier and more expensive with the f/2.8, but you lose the IS support. You can go wider and much more expense and very unique images in the EF 11-24 f/4L, but the IS version is the sweet spot of sharpness, focal length, size, weight and cost. The 17-40 f/4L is great image quality for a lower L-series price, but again you lose the IS support, if that's important for your use-it-everywhere needs.
I agree with this, there are some really great wide angle lenses on the market for your 6DII but probably the best value for the money is the 16-35 f/4 L, not only does it have IS but also it has a threaded filter ring which makes the Canon wides a bit unique in the wide angle offerings. Unless you are shooting low light f/2.8 is unimportant for a wide angle lens, if you are doing landscape which is by far the most common use for these lenses you will be stopping down for increased DOF so the f/2.8 really is not a factor. You can buy a Sigma 14-24 f/2.8 which is bigger and much heavier that you can buy a Haida rear adapter for that will allow the use of their ND filters which comparatively are very inexpensive as compared to the cost of 77mm screw in filters. I have both lenses and although if you pixel peep the Sigma may edge out the Canon it is not something that you would ever see in just looking at photographs. The Canon for you in my opinion is still the better choice, I shoot with a Canon mirrorless and have the adapter with the polarizing filter in the adapter, for you to use a polarizer on the Sigma or even the Tokina which is also a great value you would have to buy a large square filter adapter system to accommodate the polarizer that would run hundreds of dollars. Lastly the Canon does not have the large bulbous front element, that is why it can use screw on filters, but this also helps to reduce flare when shooting towards bright light sources such as the sun.
Another great choice is the Sigma 12-24 Art, or even the non Art version, it is clearly wide enough to use on both of your bodies but then you will not only lose the IS but also the ability to use screw on filters.