How do I choose what format Outlook uses to reply/forward a message?

"You can edit the original email and change the format to the format you want the reply or forward to be in. This is better than changing the format after hitting Reply/Forward as it keeps all of the existing formatting that is otherwise lost once you hit the Reply/Forward button.

Under the Message Tab select Other Actions->Edit Message (This can be added to the Quick Bar). This gives you the Insert/Options/Format Text tabs for the current message. Then under Options select HTML/Rich Text, and then Reply/Forward as desired."

"I added a shortcut to the Quick Access Toolbar to ""Clear All Formatting"". Then, when I reply/reply all to any email, I can select all the text in my reply and simply click the button to format exactly as my default style set.

You can add this shortcut to the Quick Access Toolbar as follows:
Reply/Reply All to an email.
Go to the Format Text tab.
Click on the Styles drop-down icon (bottom left).
Right-click on Clear Formatting.
Click on Add to Quick Access Toolbar."