If your experience is mainly in Python ecosystem Django is just hard to beat. If it's Java based then things are more complicated I think - these days I have been avoiding heavyweight back ends and using minimalist stacks of which Micronaut [1] is my favorite. I use it with Groovy (though it supports Java / Kotlin), Postgresql as database and combine it with VueJS on the front end for a very agile but highly performant and robust stack.
As someone coming to modern Java from the Python world, I've been pretty happy with Spark (no, not that Spark - it could really do with a more unique name): http://sparkjava.com . It's very simple, doesn't require any of the Spring madness, and looks a lot like Flask in its minimalistic approach. It has a (pretty rough) Kotlin version.
I've also dabbled with Ninja (http://www.ninjaframework.org) but that was ultimately disappointing. It relies heavily on dependency injection, which is not my cup of tea.
Thanks, Spark looks great! Very much like Micronaut actually. The only thing I see missing that I would want is handling of async ... not sure if I'm just missing it in the docs.
