Ninja is pretty cool. It works well with Kotlin, which is what I reach for today with the JVM, but tbh the easy path today is pretty much Spring Boot so I have some trouble rationalizing using anything else.
My main reservation with Spring is the heavy dependence on runtime reflection. I'd like to see a Java web framework designed for conventional server-rendered web applications (as opposed to API backends), with authentication, CSRF protection, form validation, and so forth, but without heavy use of runtime reflection for wiring up objects.
Edit: Also, especially given that Project Loom is on its way, I think a blocking API like Ninja or Spring MVC is best.
Add Phoenix style LiveView and we'll have a winner..
I think it is much cleaner to have a clean separation between server and client frameworks and use, for example, something like Quarkus on the server and Angular on the client and get the best on each side instead of a strange mixture.
I go back and forth on whether that's a good architecture in principle - a client-side UI calling an API has clear technical advantages, and it's much easier to build first-class applications that can run in JavaScript than it used to be. But certainly if you want that architecture then I think Wicket is still the best way of doing it - indeed I'd say it's the best UI framework I've used anywhere, the model/component split forces you to clearly decouple your data from your UI in a way that's not as thoroughly enforced in e.g. Qt.