Seriously? That has to be Tab. Between indenting code, cycling windows, moving between form input elements, switching tabs in applications with tabbed interfaces, and innumerable other uses, it gets a lot of work for vital things.

On a less serious note, as a devout vi user, I was tempted to vote Esc.

And on a boring note, the space bar wins hands down for most used key. Itsmerenecessitymakesitaformidablecontenderinthispoll.

Makeshifts last the longest.