Upto what extent Selenium Webdriver is best to test the functional Testing of web application?

1)first depends on duration.

2)selenium webdriver is good for execute regression test cases for web application but 100% manual testing(functional testing,integration testing,system testing, retesting) should be done for web applications  

Wثلا is used for communication between two or more people by working in the company or in two different places of the web will not be easy to work in the shortest possible time

it depends on how your web application is developed.
In case if you are using lots of silverlight application then Selenium is not for you.
For everything else, Selenium is the best bet.
You can choose any of the language you want and write your own customized scripts.