Too many variables. If I'm going solo and it's not too cold and there are a lot of (the right kind of) trees, I like the hammock approach to allow camping away from established sites, and/or to just hike until darkness and stop when two trees are found.

When not a lot of rain or otherwise bad weather is anticipated, a very light bivy and poncho tarp are a light approach that I quite like. Or in winter, a better tarp and beefier bivy sack.

In a dry climate where bugs aren't anticipated, dewfall and rain are rare, it's great to carry just a poncho as backup rain gear and shelter both, but mostly just cowboy camp (as you say "just a sleeping bag".

Otherwise, some sort of tent, and the right tent depends on anticipated conditions too --- by default for me a light single-walled tent, but there are exceptions ...

I just don't see a one-sized fits all. If I could only own one shelter I suppose it would be a light tent.