Linux

Hm. Well, I've answered in angry detail in a response to someone else, but as evidence that I'm not some loner, I present you with:

• Mercurial, for which it's hard to find usage estimates
• Pijul, the spiritual successor to Darcs, much younger that git; also hard to find usage stats
• Jujutsu, an interface currently on top of git, as an attempt to make git suck less by giving it a more Mercurial-like interface (or, "workflow" since a large number of people seem to be struggling with the concept that the set of commands of a CLI tool is its user interface). This one's a little easier, because we can see it's got 12k stars on github.
• On the Jujutsu README is a link to a list of "other tools trying to solve similar problems", the "problems" being how much the git UX sucks. That page mentions git-branchless, sapling, gitless, and sturdy -- all front-ends to git with less bad UIs.

forget that, he said UI.