sourcehut. I like how it’s structured, where issue trackers, repos, and so on are independent of each other but can be grouped using a project, and you can have as many of each as you want or none at all. You should be able to have a huge monorepo with many issue trackers, or a single issue tracker for a project split across many repos if you want. GitHub doesn’t really allow you to do either, certainly not the former, and same with most of the alternatives. Everything else seems to clone GitHub’s workflow for contributions as well which I can’t stand (sourcehut uses git send-email as the primary contribution method — but there is also a GitHub style PR button —, which apart from the email jank I find much better because once it’s set up you can just send changes to any project with just a local clone; it also means you don’t even have to be registered on sourcehut to send changes to a project hosted there).

I also self-host cgit I suppose but that’s not really a GitHub alternative.