If you're looking for the answer to which lens you should buy, this probably won't tell you; however, it might help narrow the field. I have spent many years now travelling the world with camera in hand. That camera has been a variety of different brands and sizes, with various types of lenses, but after all of this time, my preferred travel setup is a full-frame DSLR camera with a 35mm lens. Why, you might ask? Well, there are a few reasons.
1. Image Quality
I shoot with a Canon 5D MkIII at the time of writing this. It is one of the best all-around cameras I have ever used. The image quality is sensational, the lens selection is second to none, and the low light sensitivity allows me to take pictures in situations I had never dreamed of previous to this camera.
The Canon 5D MkIII is not the smallest camera, but it's not huge either. Combine this with the Canon 35mm f2 / IS USM lens and you have a relatively small setup that still allows for some great incognito shots in low light or otherwise. When compared to Canon's own f1.4 lens, the f2 may not be optically superior, although it holds its own and takes fantastic pictures, but it does offer a size advantage in that it is considerably shorter off the front of the camera.
Travel is about capturing scenes. Some of those scenes have to do with landscape, some have to do with close up stories. With a 35mm lens you can do both. I found when I tried to shoot with a 50mm lens that it was often too tight to capture the expanse of the scene I was trying to show. The 35mm is just enough wider to allow for more width at a close distance. For a close up it is easy enough to walk a little closer. This in itself changes the relationship of the photographer with the subject, and can yield vastly different results. Remember, anyone can take a photo from a hundred feet away like a tourist. I find the 35mm lens forces me to change my angle and perspective on things as I can't simply grab the shot like a sniper in the distance.
On my last trip to Europe the 35mm was mounted to my camera the whole time. I switched off to my 24-105mm lens a couple of times, but that wasn't really necessary. So there you have it, my unequivocal "if I could only take one lens with me to the island which one would I take" lens is a 35mm lens.
For Canon full-frame shooters you have several choices of 35mm lenses:
This is my personal favourite. Medium price point, great optics, small-ish size, stabilizer for night shots and video.
Canon 35mm f2 (No stabilizer)
This is a cheaper alternative to the lens above. It's not quite as swish, but good for those on a budget, or trying to decide whether they like shooting at 35mm. Bonus points if you can find the older version of the lens that has a far superior build quality to the cheapo plastic version in production now
This is the best Canon 35mm lens hands down, optically speaking, but when it comes to travel I find it a bit large. However, the pictures from this lens are stunning. You really need to be committed to drop this much money on a 35 though!