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.
6D, 5(film)x2, 650(film)x2, Canonet QL19, EF 15/2.8, 24/2.8 IS, 50/2.5 & 70-300/4-5.6L
Fotodiox Pro OM/Eos adapter, Olympus OM-10, Zuiko 24/2.8, 50/1.4 & 135/2.8, Ensinor 28/2.8, XA/2, Mju 410
Apple iPhone 4S & iPad 2. I've only listed the equipment I still use for photography!