if you want to build blazing fast, modern, beautiful and progressive web app gatsbyjs is your best choice.i have walked through many open source applications like nextjs, reactpwa, and others but found gatsbyjs awesome in terms of frontend only framework, 

Modern stack without effort:

The big benefit of using gatsbyjs is that you get full working latest technology stack without any effort, everything is already set up (react.js, Webpack, modern JavaScript and CSS ) and you just need to write your business logic and start development

Progressive web app

Gatsbyjs is progressive web app by nature it loads basic UI very quickly and makes API calls later that improves usability, it also prefetches urls that improve site performance

Fast performance

Prebuild pages boasts performance of web app, gatsybjs does not generate on run time on request. Instead, it prebuilds pages that are ready to deliver instantly

Static scalability 

gatsybjs is just a static website that you can place on any web server or s3 and server by cloudfront etc. it relaxes you from the complex deployment of backend and databases.

Bring your own server

you can make  you API server separately and use it with gatsbyjs , it has the awesome feature of graphql which helps to connect any kind of backend