I'm not a pro, but for a few years now I've used a Sony a7S with just a single lens: the Sony / Zeiss FE 24-70 f/4:
It's dust and moisture-resistant, although I don't really put that to the test since the a7S itself isn't, so I keep it out of the elements. I mostly shoot landscapes so I'm usually in the wider end of the range, but I've found that 70mm is usually plenty of zoom to let me snap a closer shot of something I can't walk to.
I like traveling light and being able to fit all my camera gear in one tiny bag packed into a larger bag, so this lens has been pretty much perfect for my needs.
I'm really happy with this shot of Gullfoss in Iceland (70mm, f/11):
Here's a quick snap of my brother and sister-in-law from that trip (46mm, f/8):
And one of @Felicity (70mm, f/5.6):
And how's this for low light? (24mm, f/8, 30s exposure)