Regarding the title of the post "Best lens.....", it really depends on what "best" you are after.
If its the best priced lens, then, the Samyang would fit the bill. But, its suffers from a lack of edge sharpness (coma) and it has some subtle funky distortion involved (mustache distortion).
If you want the sharpest lens, then a Zeiss, but the price can be a killer.
Personally, I bought a 15mm Zeiss for this type of work, as it's wide (the sky is a big piece of sky-scape), and sharp.
There are some 3rd party lenses that probably fit in between, but I'd always hire and try before buying, IMHO.