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.
