For the 6D I bought a new lens: Canon 17-40mm f4L. I’m planning to use it for landscapes mainly (it’s very big with the grip and intimidating for street) and I didn’t have a wide angle FF lens in stock.
With the cropped sensors, I used to use the Sigma 10-20mm which changed my life at the wide angle end (and also gave me headaches because of the distortions). But I can’t use it on FF sensors. With the 5D I was using the Canon 24-105mm f4L, but it’s very long even at the wide end for landscapes, so I was never happy with it.
It’s not the 16-35mm f2.8L, but it’s a fraction of the price. Especially on eBay where I got it for just over 200€.
I’ll still use the 24-105mm f4L on the 6D, but not for landscapes. Portraits and street until I get a Canon 85mm f1.4L for portraits (they’re still expensive second hand).