If I leave the house with just one lens on the camera and nothing else then it's usually the P25 f/1.7... although it is sometimes my O45 f/1.8.
I does depend on what you want to photograph and where you intend to visit on your travels, but from the lenses I have, if I was travelling with just one lens then it would be the P25... just about wide enough for most use cases, just about sharp enough for good crops should you want to 'zoom' into the picture, and just about fast enough for most lighting conditions... the M43 equivalent of a nifty fifty!
Like others have said, it depends where you're traveling. In cityscapes, I often take my Pen-F with just the Olympus 17mm F1.8 lens. If I were traveling somewhere that's a bit of a mix and am taking my OMD E1 Mk II, then I'd probably put the 25mm F1.2 lens on it.
I have done it, and either a 28mm-equiv or the Panny 20mm makes for a versatile single option. The latter is a good choice for getting in between wide angle and the standard fifty. ~40mm gives you something of both.
I shot most of a whole year with just the Oly 17 f:1.8, I like it a lot and it would have been my choice for an everyday lens. Am a bit leery to the "one prime for travelling" though. Depends on where, what and why.
For me it's the Olympus 17mm 1.2. I can do anything with a fast 35mm (or 24mm) equiv lens (Street, portraits, cityscape, low light, close ups etc). The 12mm f/2 is really good for travel too. I like to travel with only one prime; you need to be precise with your composition. I truly think that make you a better photographer . So a 17 f1.2 or a 12f/2 on a Pen-F, 2 batteries and I'm set.