A recent thread here on the board, the poster was asking about why her images were somewhat distorted more than she anticipated with her wide angle lens. She was using a 10-20. There are several wide angle lenses by Sigma, Pentax and Tamron, in a number of ranges and speeds.
They all do the various aspects of wide angle shots ranging from superior to OK. No one single lens does everything at a superior level. For instance the wider the lens, the more distortion you are going to have on the edges and corners. None of the wide angle lenses are inexpensive. So, other than saying - I want/need the widest, I would suggest that you consider; distortion, focal length range, weight, speed (constant aperture), flare resistance, etc., so that when you acquire the lens, you know and understanding what you are acquiring - the strengths and weaknesses. This is more desirable than getting to where ever (say - Scotland), and coming back with images that you may be dissatisfied with due to something that you did not consider.
I have seen images from all the lenses that are striking and wonderful shots. Even with wide angle lenses, you can stitch even wider....