If it's for future employment, then Spring, Ktor, maybe Micronaut or Quarkus.
If it's for 'fun', and to learn about HTTP/GRpc/REST/GraphQL/whatever else, then pick a lighter weight framework, OR if you really want to get deep, look at what these frameworks use under the covers.
Spring, and other frameworks, are very productive because they hide a lot of stuff, and auto-configure, and... but like any framework, when things go wrong, it can be much harder to figure out why...