Which one is most useful, you ask? Well, it really depends on what you want to accomplish.
But in all honesty, your question is like a carpenter asking which tool is the most important for him to have in his tool box. If all he has is a hammer, after awhile every problem will look like a nail to be pounded. But he also needs wrenches (many sizes), screw drivers, etc.
Learn Visual Basic, C-Sharp, HTML, ColdFusion, JavaScript, and so on.
For windows programs, even that depends on what you are aiming for - gaming software? home financial software? business software? And in business software, you have inventory-tracker software, client-tracker software, financial software, etc.
Learn them all, and that way what EVER direction you end up taking, you'll be prepared.
Good luck.