My default tends to be Ansible because it is really versatile and lightweight on the systems being managed. That versatility can bite you though because it's easy to use it as a good solution and miss a great one. Also, heaven help you if you need to make a change on 1000s of hosts quickly.

I also use (In order of frequency): Terraform, Invoke (Sometimes there is no substitute for a full programming language like python), Saltstack (1000's of machines in a heterogenous environment)

If I were going to deploy a new app on k8s today, I would probably use something like https://github.com/fluxcd/flux.

I haven't really had a pleasant time with the tooling around serverless ecosystem yet once you get beyond hello worlds and canned code examples.