Poll: What is the most useful keyboard shortcut command of all time?

If you had to pick the most useful keyboard shortcut command of all time, irrespective of operating system or application, what would it be?

In this week's TR Dojo video, I show you how to create custom Windows keyboard shortcuts with the free AutoHotkey utility.

While the keyboard shortcuts we regularly use often depend on our operating system and applications, some shortcuts are ubiquitous--like Copy and Paste. If you had to pick the most useful keyboard shortcut command of all time, what would it be?