Replying to the OP's initial question - I have no experience with the Ziess 21mm, so I can't comment there.
I really like compression in many landscapes (and have a particular interest in landscape photos in the portrait-orientation, where that compression can also work beautifully in many compositions). Eg when I lived in Europe and visited Switzerland, taking photos of the Alps with details closer to the foreground ... magic!
Actually, I don't own a FF (my Canon DSLRs are 7D and 350D). My favourite landscape lens is probably the trusty Canon 15-85mm. It's got great IQ, and the USM AF and IS are very handy too. But perhaps more important than AF and IS is the focal range... from 24 to 136mm in 35mm format equivalent.
I find that 15mm on an APS (24mm in FF) - works VERY well for me for most landscapes. (I definitely really like those few extra mm compared to the 18-XXmm or 17-XXmm lenses). And 85mm on the tele-end is important for me (I find 50/55mm too short for a tele-end walkaround).
Though I do have an UWA (Sigma 10-20mm) - and it does come into its own in certain situations (and yes, there is a huge difference between 10mm and 15mm) - often I take my favourite landscapes around 15mm. I really like the flexibility of a zoom when out bushwalking, sight-seeing, etc (I currently live in Australia, but have lived and travelled extensively around the world).
I'm not planning to upgrade from APS-C (looking forward to what a new 7DmkII may have in store for us). Hope the OP will find the right lens for his/her needs.