The 23mm f/2 on my Fujifilm x100s, because it's fixed to the camera.
Seriously though, when I was still using my Nikon DSLRs, my most used was the nikkor AF50mm f1.8, tiny, great all-round, the nikkor AF-180 f 2.8, wonderful portrait lens among other things, and a really old manual focus nikkor 105mm f2, fantastic patina.
As my neurological problems got worse, I gave all my gear to my son and got the X100S. I really love how the lack of zoom forces me to think differently on composition, and "zoom with my feet".