Building web apps means that you need a front end that the users will interact with. With Domino the answer was XPages ( or for the hardcore old school developers you could use forms and views and pages with passthru html ) but when moving to a new development stack XPages is no longer an option so what can you use…
So before you decide on the front end technology you do need to decide on whether you want to do server-side rendering or client side rendering. Server side rendering can result in more security for your data as the backend rest calls are not exposed to the client in any way, the client only receives the data after it has been retrieved and processed by the server, however for a very busy application your server needs to be powerful enough to do all that processing so it can return the rendered ui to the user in a timely fashion.
For client side rendering all the processing is done by the client browser application. It just gets a raw stream of data from a REST endpoint and then renders that for display. The server no longer has to spend processor time on rendering of UI and data, however this does mean that the client needs to know about these REST endpoints so they have to be exposed to the outside world so there are security considerations to be made.
Server side rendering also has a number of frameworks to select from but they can be quickly narrowed down depending on what you selected as your main backend, if you are using .Net for the backend then you are probably going to select ASP.net for the UI. If you picked Java then you might use JSP or Apache Velocity, Apache FreeMarker, Thymeleaf, JTwig, to name just a small few. Other languages have their own templating engines that they use.
For our future development we decided on using Server Side Rendering with the Thymeleaf templating engine. As we are using Spring Boot for the backend Thymeleaf makes an excellent choice. There is a Thymeleaf starter project for Spring Boot which will pull in all the dependencies you need automatically and Thymeleaf itself has some specific Spring integrations also.
If you, however, are heading down the Spring Boot + Angular path then make sure you also check out JHipster. This is a code generator that can help you get up and running with very quickly by answering a few questions and then importing the generated code ( for frontend and backend ) in to your IDE.