Depends on the use case but XML is good for markup—especially if you need extensibility.

For config, Nickel & Dhall take the cake for being typed & having LSPs so the configuration writer can get immediate feedback about possible options (while eliminating invalid states) without requiring the manual—with configuration readers not needing to mess around with marshaling their types. Both these configuration languages let you import files & write little loops to make your config more DRY & makes maintaining large files (like say Kubernetes) easier.