While this may be subjective, I, and many others, have found Angular to have the steepest learning curve. There are many Angular-specific concepts, such as their digest cycle, that you must understand before you can be productive with Angular. Angular also uses a lot of tools that developers may not be familiar with, including:

• TypeScript: Provides static typing to JavaScript
• RxJS: Allows you to write functional reactive code
• SystemJS:  A module loader
• karma:  A tool for running unit tests
• Protractor: An E2E test runner that allows you to run tests that interact with a real browser

Although each of these tools brings a lot of value into the application, it no doubts adds to the already-steep learning curve for Angular.

React, on the other hand, is just a view rendering library, and so is much easier to understand. The basic idea is that you create components, pass in some inputs, and React will generate the final view and render it onto the page. You can arrange these components in different ways and nest them inside each other, as it's all composable. You may have to learn about the difference between states and props, and also the lifecycle methods, but that can be done in a few hours at most.

Perhaps what people are referring to when they say "React has a steep learning curve" is the ecosystem around it. The React ecosystem is organized in a way where you have many tools, each doing one specific thing. This is generally a good thing but it also means you'd have to spend the time to pick from the different options, and perhaps spend even more time debugging incompatibilities when you try to integrate them.

For instance, you may use React Router to route your pages. You'd need to learn Redux or MobX to manage your state. Most of the time, you'd use Webpack to bundle your application. However, many React developers also use libraries, such as ImmutableJS, Flow, TypeScript, Karma, and ESLint, which are not compulsory tools, but can often confuse new developers.

An alternative approach is to use a full-featured boilerplate, such as React Boilerplate (reactboilerplate.com), which has a shallower learning curve, but you'd still have to learn the conventions used by the boilerplate author. Furthermore, if there's a bug/issue with the boilerplate, it'll be much harder for you to debug.

In terms of concepts, React is much simpler than Angular. Even with the React ecosystem, the learning curve is still manageable. Personally, having to stitch your own stack together forces you to understand what each tool does, and how it interacts with other tools, which is a good thing.

Vue.js boasts an even simpler learning curve. It does not use JSX, but a simpler template-like syntax with its own domain-specific language (DSL). It does not require Webpack and developers can enable Vue.js just by including a typical <script> tag.

<script src="https://cdn.jsdelivr.net/npm/vue"></script>

Therefore, it's easier for developers not using a framework to migrate to Vue.js, as they can more easily convert their HTML into HTML-like templates, and can incrementally adapt the entire application to Vue.js.