I don't think you can go wrong with the 16-35 f/4 IS. Along with the 24-70 f/2.8 II and 70-200 f/2.8 IS II, I think these are the most amazing zooms on the market.don't forget 200-400
I can just add to what Macguyver is saying. I have the Zeiss 15mm and 21mm and they are both phenomenal lens, especially the 15mm, which I totally fell in love with it this summer. But the 16-35 f4L IS is a lot more versatile and the IQ is very impressive. Be also aware that the Zeiss 15mm has a 95mm filter size, so filters are not exactly cheap.
for me, the main problem, there is no way right now how to test Zeiss 15, it could be excellent lens, but I don't like the idea of buying lens for 2,5k euro before trying it