Vue.js / Nuxt.js
Vue is my go-to framework for building user-interface and front-end wizardry. I've worked with other frameworks like jQuery, Angular 1.0 and React but I got stuck with Vue because it is just so easy to get started with. It does not have a huge learning curve like React and Angular 2.0 and I personally would recommend you to try it! Once you the gist of it, proceed to nuxt! This site is built using Nuxt.js
For fast building of websites, this is my go to CSS Framework. It is responsive out of the box and it comes with fundamentals UI components to get started with. Specifically, I would use BootstrapVue to compile it together with Vue.
Tailwind CSS (new)
This is a new framework I love working on right now. It has no components, no forms and no UI. So what's good about it? Everything! It is a utility based framework which means you don't have to write a single CSS class with it. You can build your components however you want. You don't have to overwrite existing components cause, it doesn't have any. This site is built with Tailwind CSS.
I love this. You can define variables and write for loops when building a css file. AMAZING. I usually use sass if I am customizing a default bootstrap layout. It is way easier and cleaner to re-build a whole bootstrap css rather than creating a new file to overwrite the default bootstrap css.
For building full-scale web application, this is the chosen one. It has a really powerful templating engine called blade. As a bonus, it has vue installed out of the box! Wee.