It can be from a song, or just a solo.

My favorites in order are:
1- Jimmy Page's live solo in stairway to heaven on the cd "How the West Was Won" (by the way, if you like Led Zeppelin, that CD is a MUST buy, no joke).
2- Jimi Hendrix playing his song "Little Wing" in crazy solo form. For that matter, anything by Hendrix.
3- Slash in the Velvet Revolver song "Slither".
4- Black Sabbath solo in "Ironman"
5- Slash in the Guns N' Roses song "Sweet Child of Mine"
6- Metallica solo from "Seek And Destroy"
7- Compilations of little bits of guitar from Queen.
8- ANYTHING done by Eric Clapton.

Well there is my incredibly slim list for you.