Sigma 12-24mm?
But then again it depends what you mean by landscapes. Some interpretations can lead to incredibly narrow views, think leading lines, urbanscapes etc, whereas others show herds of gnus drifting majestically across the plains. Effectively you could be using anything from 12mm to over 500mm.
Having said that, if it's rural landscapes, then you are probably better off going for something above 35mm to avoid distortion, although it can be corrected in PP or used as a feature.