I don't think guitar solos should count here since they are a different subject altogether, but I have always thought the outro to Boston's Peace of Mind
http://www.youtube.com/watch?v=OaR2JeqxQDY was a masterpiece of multiple guitar harmonizing. It still gives me goose bumps if I haven't heard it in a while.
Funeral for a Friend (kind of the intro to Love Lies Bleeding), by Elton John is also killer.
http://www.youtube.com/watch?v=5GYI6XJH9Ss