The Sigma 20mm F1.8 EX DG ASP RF
If you specialize in landscape photography, architecture, or photojournalism of any kind, the 20mm F1.8 EX DG ASP RF is a great option for your kit.
The coolest feature of this lens is its incredibly fast maximum aperture. The F1.8 aperture makes this the fastest super-wide angle lens on the market. Photojournalists, in particular, will find this to be a life saver in low light situations.