Zeiss 15 is definitely better, if you don't need AF, but they both can't beat Samyang 14/2.8 UMC, for the price 
within reason I would select the best IQ and let price be a secondary consideration. How is the Samyang IQ and will it stand up to usage
Canon 14L'II
- soft corners, high aberrations, no front filter, unpleasant bokeh (in my opinion)
+ dust-moisture-proofed, relatively low distortion (for this lens class), AF
Carl Zeiss 15/2.8
- bigger and heavier than other 2
+ sharp, relatively low distortion (for this lens class), low aberrations, great focusing ring, 95mm front filter, decent bokeh
Samyang 14/2.8 UMC
- strong and complicated distortion, no front filter
+ sharp, low aberrations, great focusing ring, low price, decent bokeh
I'd say that Zeiss 15 is the best of this 3, but it is the most expensive. Samyang 14 - best price/performance. Canon 14 is not worth its $.
P.S. If you don't care much about size/weight, high price, AF, slightly narrower angle and aperture, then maybe Canon TS-E 17/4L is what you are looking for. 17L would be on the top of my list if I could afford it
.
