Thought I'd chime in here too as I'm in a similar situation looking for an ultra wide angle of decent quality. I too have the 16-35L, 24-70L II, 24 tse II and a Zeiss 35mm f2.0 for landscape work but have been looking for that ultra wide lens for greater FOV without sticking a pano.
I have rented the new Zeiss 15mm f2.8 and it is spectacular. I didn't care for the ergonomics of the 96mm front filter size. This lens is a monster and expensive. I just couldn't justify the cost. If price is not an issue, this Zeiss 15 is hard to beat in IQ and corner/corner sharpness.
As for the other Canon's 14L and 24, I've never used the 14L and cannot comment but the 24 i've had and sold since I purchased the 24 tse II. The 24TSE II is extremely sharp and lightweight in comparison to any Zeiss lens.
One of the big differences, IMHO, is manual focus vs AF for portability and getting critical sharpness. Anytime I'm going to use manual focus I always set up on a tripod and X10 the image in the VF. Obviously, any shot OOF is a waste of time and energy.
Best of luck.