I think it’s YAML.
I’m not happy that it’s YAML but it’s become ubiquitous. Sure, there are lots of other formats that others have mentioned, but I’m sorry most of them are positioned as “it’s better than YAML!” and the fact that everyone is mentioning YAML, even if it’s about the things it does wrong (and boy does it do things wrong) still means that YAML is on everyone’s mind.
I’ve been using ConcourseCI because another team manages it and it is integrates with Hashicorp vault.
It’s good. The big advantage is I don’t have to deal with maintaining it.
The downside is their story on GitHub integration is kind of crummy, you have to create your own web hooks because our ConcourseCI instance is too busy for poll based resource checks.