STAR WARS REBELS: Debut of Darth Vader - Clip
Because Star Wars Rebels is a few years before A New Hope, it's possible for a lot of our favorite original trilogy characters to make guest appearances. R2-D2 and C-3PO were the first of the classic characters to make a full guest appearance, and Bail Organa was the first prequel character to appear. During the ABC premiere of Rebels, Darth Vader made a kind of cameo appearance, making the first of likely more appearances on the show.
This video was shot by a camera pointed at a TV screen, so the quality leaves much to be desired. I'll update it with a better one once I find one, or you can email me if you find a better one.