What is the fastest and most efficient programming language for GUI?

Jakie,
This topic is always a good source for spirited debate. When asking this question, you should first consider whether this is a web based, desktop or mobile GUI. All three have a few different heavy-weights with their respective strengths and weaknesses. When I say weaknesses, I'm referring to the trade-offs and/or compromises you make due to the limitations inherent in the structure of that particular language or framework.

Framework? Instead of thinking about which language to use, you may want to consider which framework best suits your needs. For example: web applications are set on fire by using the Rails framework for Ruby or the Django framework for Python.

If you are talking about a desktop application, then .NET, wxGlad, JavaFX, Adobe AIR, and a few others should be considered. .NET will obviously be your "goto" for applications exclusive to windows. wxGlad certainly has traction as Python's desktop GUI framework of choice as it has fully implemented the enormously popular open source 3D animation studio, Blender. JavaFX and Adobe AIR, in my opinion, are more suited for those who would like to say in the HTML/CSS web based GUI world but want to use it as a stand alone desktop application.

Mobile applications are basically device dependent still so choosing the correct framework/language is basically dependent on device compatibility.

No coding language is going to be efficient if you do not understand programming "good practices" such as MVC, DRY and using version control software like SVN and GIT. From a single freelancer to a team of developers, these principals scale at any size to make your coding life easier.

I hope that helps!