Gitlab.com just started doing shady stuff and requiring phone numbers or something on sign-up if what I read a few days ago here, is correct. For self-hosting the software should still be alright.
Github.com is by Microsoft and not free software. I don't know what direction Microsoft is taking with it, but it is widely adopted and they give you free CI and other stuff.
Codeberg, Sourcehut etc should be fine. I haven't heard negative things about them.
"Best" is running my own Forgejo on my server. At least that's what I think. But I also keep things on github, since all the people are there.
It is git. You can fork repos. And some platforms can mirror a repository and keep it synced. If not, you'd need to build something with webhooks. Or keep both synced manually every now and then (or on a new release/tag.)
From personal experience, if you're hosting Gitlab and make it available to the internet, make sure to keep it updated or your server will be super slow hosting a crypto miner within a year.
it’s built upon/by the same devs as forgejo, which is open tech to self host your own git server (with federation potentially coming), so supporting one supports the other
Codeberg is a German non profit iirc, I host some stuff there but most is on my personal Forgejo.
The Forgejo devs (mostly centered on Codeberg) are also the ones pushing for federalized code forges (I open a PR on your git server from mine and so on)