I used the System Preferences > Keyboard > Shortcuts > App Shortcuts Preference window dialog.
I added a new shortcut (using the + widget) for All Applications, typing in "Zoom" without the quotes, and used "Control-Z" shortcut. It's one of the first things I do when I set up a user account. Works great - try it, you'll be hooked. Set it and don't forget it. The shortcut becomes a toggle for the Window > Zoom command.
For Office, Zoom is replaced by Zoom Window, and use the same shortcut.