Anything but YAML
After a run-in with YAML for CI purposes in the context of Travis, I can't argue with the view that the use of YAML has all the appeal of sticking one's hand into a meat grinder. Heavens know that after the Nth cryptic YAML parse error from somewhere in the build pipeline I wanted nothing more than to rage quit.
While I had some not unpleasant experiences with the Groovy-based DSL for an automation plugin with Jenkins CI, I'm not convinced that Kotlin is more suited somehow. To me it feels more like a 'We use Kotlin because we created it' kind of deal, but that's something one has to find out in practice, I guess.
Either way it's not YAML, which is already a sole reason to use it over YAML :)