Auto HD For YouTube is a extension for Google Chrome, allowing you to automatically set your video quality to whatever default setting you choose, and more.
To change the settings for this extension, go to your extensions page and click on Options under Auto HD For YouTube. You'll see that you have the ability to set your new default resolution.
You can choose from the standard YouTube formats: 1080p, 720p, 480p or 360p.
Once downloaded and installed and you tweak the settings, you never have to mess with it again. You will probably forget you have it installed.