Maybe the recent Canon 35mm f2 IS -- for FF and standard sensors.
I use the older 35 f/2 on both cropped and FF and it is very versatile. light, small, sharp-enough, close focusing distance, and very affordable. I'm on my second 35 f2. I'd like a better build, but can't justify it's replacement.
I do not make money from photography, so size/weight are important to me. I want to have fun when I'm out.
on APS-C: my goto travel/one-lens solution was the tamron 17-50 f2.8.
on FF: have not found it yet(maybe the older brother 28-75, every other zoom is heavy)