If I was to only carry the one lens - the one on the camera - it would be a 35mm for full frame.
My current travelling light set up is different to that - I have a 50 on the camera, and a wide (24) in a bag or pocket somewhere. 50 is great for forcing a tight field of view, but there are times when wider is needed.
100D, 6D, 5(film)x2, 650(film)x2, QL19, 18-55 STM, 15/2.8 fisheye, 24/2.8 IS, 50/2.5 & 70-300L, Fotodiox Pro adapters
Pentax S1 & 55/2 Olympus OM-10, Zuiko 24/2.8, 50/1.4 & 135/2.8, Ensinor 28/2.8, XA/2, Mju 410
Apple iPhone 6 & iPad 2. I've only listed the equipment I still use for photography!