First question has to be Budget?
Then, my suggestion would be to head for a decent sized retailer who may stock a few options and try them out for yourself. Some ultra wides introduce heavy distortion (not ideal for landscapes) while others may display chromatic aberation (purple fringing in areas of high contrast). Some copies of some lenses may have problems (as reported with Sigmas). Try and test the copy you will be leaving the shop with, that way, you'll be buying a known quantity and if it develops a problem later, you'll be able to return it to the shop and get the problem sorted far more easily than trying to get satisfaction at long distance.