What is the most common used keyboard shortcut?

It depends on what the computer is being used for. For general typing of letters of essays, "Space" or "e" are probably the most used. For a gaming computer "w" (walking forward in most games) is often the most used key by a large margin, followed by "a", "s", and "d" (left, backwards, and right is the WASD control setup). Computers that run other specific programs a lot might have another key, such as "Enter" that is used the most because it is associated with a specific function.