If you simply want to serve a web ui, Play and Spring Boot are good. Play is a bit more self-contained. It provides a lot but doing something a bit out of the ordinary can be challenging. Spring Boot is more agnostic in that respect but relies (obviously) on Spring for dependency handling.

As well as those 2, Dropwizard is excellent if you're writing REST apis and provides many enterprise-friendly features out of the box.

SparkJava (don't confuse with Apache Spark) is a very lightweight Sinatra/Flask-like framework using Java 8 features, and good for REST apis too.

2 more to consider IMHO: Vert.x, and Comsat from Parallel Universe. Parallel Universe have one of the most interesting technologies in Java-land IMO. They provide a library called Quasar that adds suspendable green threads (similar to Erlang processes) to your code using bytecode instrumentation. This allows a single server to handled a huge number (think 00,000s) of concurrent connections. Vert.x has support for Quasar too.

I'm sure others have favourites too.

https://www.playframework.com/

http://projects.spring.io/spring-boot/

http://www.dropwizard.io/0.9.3/docs/

http://sparkjava.com/

http://vertx.io/

http://docs.paralleluniverse.co/comsat/