For travel, I'd go with the Sigma 24-70mm f/2.8 DG DN Art. It's sharp, covers most situations you'll encounter, and the build quality is fantastic. Yes, it's a bit heavy, but the versatility makes up for it. I've taken it on three international trips and never felt like I was missing shots.

Panasonic 20-60mm f/3.5-5.6 hands down! Super lightweight, great range for travel, and dirt cheap compared to Leica glass. The image stabilization works great with my S5. Only downside is the variable aperture, but for daylight travel photography it's perfect.

Consider the Sigma 16-28mm f/2.8 if you're into landscapes and architecture while traveling. The wide angle really opens up possibilities, especially in cramped European cities or dramatic mountain vistas. Pair it with a compact 85mm and you're set.

I'm with WilliamerusY that the 24-70 range is great for versatility, but honestly, I had a pretty disappointing experience with that Sigma build quality on my last trip. I took it to Japan and the weather sealing just wasn't as good as I expected... I think I ended up with dust inside the barrel after just one week in the city. Unfortunately, the images felt way more clinical than the Leica glass I've used before too. Not sure but IIRC some people say the native Panasonic glass has better communication with the body for stabilization? I've heard that somewhere anyway. I'm kinda wishing I'd just invested in the Leica stuff from the start even tho it costs a kidney. Using the third-party stuff was just such a headache for me personally compared to the native options.