My favourite is King's Quest VI: Heir Today, Gone Tomorrow. I adore this game so very much! It's got everything that makes a game a masterpiece: a compelling story, memorable characters, magical soundtrack, gorgeous graphics and fantastic replay value. I can play KQ6 again and again and again!
While waiting for the upcoming reboot, I highly recommend getting the King's Quest Collection on Steam or GOG, and then download this patch to make the games run better; like updating the DOS version of KQ1 to SCI, the addition of the enhanced/Windows version of KQ6 with HiRes character portraits and GUI, and more.