Zeiss Touit 12mm f/2.8

Pricing and availability at B&H, Amazon

This is another one of those pieces of gear I didn't think I'd like, but after testing it I liked it so much I bought one with my own hard cash. It's a crop-sensor only lens, but this makes it light and compact.  12mm is the full frame equivalent of 18mm, which is a really wide but still rectilinear. I use it sparingly but whenever I do I get great results.