At 21mm, definitely.
But, if you need anything wider, the 14-24 outperforms the Zeiss 18mm, and is equal to the Zeiss 15mm.
I've noticed that when viewing shots(in Lightroom) of outdoor scenes I've shot with, for example, the Zeiss 21mm and Zeiss 25mm/2, and then the same scenes and lighting conditions shot with the 14-24, I have to goose up the contrast of the 14-24 shots by a very small amount to match the 'look' of the Zeiss lens shots.
Funny you mention the Zeiss 18mm:
I originally bought a Zeiss 18mm, but didn't like the illumination falloff toward the margins of the image on the 12MP Nikon D3 I was shooting with at the time. So I returned it, and for about US$60 more(in 2009) I got the 14-24, figuring that it gave me my 18mm and more FL-wise, and with it being a modern zoom, it should be more telecentric so it wouldn't have the falloff issues that the ZF 18mm had. I was correct.
The Zeiss 15 I have not used. I've seen an online published test (http://www.3d-kraft.com/index.php?option=com_content&view=article&id=127:uwa-comparison&catid=40:camerasandlenses&Itemid=2)between
the ZF15, Samyang 14, and 14-24 that showed what you have stated: Basically, the Zeiss 15mm cannot be said to be superior to the 14-24.