A lot of these answers give technical reasons, but career wise it is a different answer for me. JavaScript. Once I got a solid understanding, the ability to do

A lot of these answers give technical reasons, but career wise it is a different answer for me. JavaScript. Once I got a solid understanding, the ability to do so much more visually and interactively on our own projects, bookmarklets and chrome extensions for sales / business department, etc, that was when they really started to notice my value at my first tech job. They felt like I was a wizard, able to modify the features of almost any existing web tool they used.