one day i'd like to get a Tamron 18-400mm for my canon. it would replace the 2 kit lenses that came with the canon and offer more zoom. i would also have to get an external flash though because a lens that long casts a flash shadow with the built in lens.
For my Oly system. 12-200 with spotting scope adapter. In some situations, a 12-40 f2.8 would be a better choice. Really depends on what you want, and how you document things.
If going deep and want ultralight: Sony RX100 7, or Panasonic ZS100. Not a one lens answer, but sometimes you want a whole system that fits in a pocket. Either of these can do a respectable job with minimal gear.
When I'm going lightweight, I bring my Nikon 1 Nikkor VR Zoom 10-100mm f/4.0-5.6 for my waterproof (but not actually waterproof) mirrorless camera. That specific lens wouldn't be anyone's best choice now given that Nikon discontinued the line, but I think that zoom range on a crop-sensor mirrorless works well as a compromise for both wildlife and scenery.
18-200 for my Sony A6400 (with the crop sensor it's more like 27-300mm) but I've actually been looking to add a small 10mm prime for night photos. For the 6 oz or so they weigh, I can justify taking it. That said, I'm still not sure I will.