Modern JavaScript Frameworks

Hi! What is, in you opinion, currently the best JavaScript tool set for creating a modern web app. Which framework has the most advanced, effective and thought through API design? In general, I would like to know what patterns modern web frameworks use: is it MVC or something else? Reason I ask is because I come from a Java/JSP/GWT background and I want to learn more about modern client-side JS frameworks because with Java on the client, I always feel like I'm sitting in second row.

Many thanks in advance

If you are familiar with Java and Servlets/JSPs, and you are interested in learning about creating modern Web Applications, you should learn JSF (Java Server Faces).  JSF 2.2 is an excellent framework for building Web Apps and JSF is now part of the JEE spec--from JEE 6 onwards, so it is widely supported.  It has a lot of built in JavaScript/Java libraries you can use to create your frontend, and has even more third-party libraries, PrimeFaces, IceFaces, RichFaces, etc.

If you are dead set on using a JavaScript framework, ExtJS is really good; it is pretty much entirely based on the MVC design pattern.  NetBeans has support for ExtJS, which makes development easier.

Angular, Ember, Backbone, in addition to numerous templating engines (dust, mustache, etc).  If you don't already know JQuery you definitely need to learn that.  A quick Google search will tell you a lot on this particular subject.

React.js and Flux from Facebook is a really interesting take on web application architecture