Pretty much everything I carry is a single body, and single lens combo, never go out with more.
Currently its my M3 and 50mm Sonnar-C, and will be for the forseeable future. Simply nothing I cannot shoot that I want with this lens, or in any light when twinned with iso 400 film (and an ND filter for when I want to use wider apertures outdoors). A 1st version 50mm summilux might interest me one day, just out of curiousity, but for now I very much like my sonnar, especially the size and wide aperture and simply don't shoot close enough subjects to notice any focus shift issues.
Other one camera, one lens options I have used or liked include an M4 with canon 35mm f2, Nikon D300/ D80 with 35mm f2, Rolleicord VA with its 80mm lens or a Rollei 35T with its 40mm lens.