You mean classic like classical music or classic rock? I´d guess on the second and i´d say freebird is a solo every classic rock guitarist needs to know!
You could do a whole lot worse than the solo from Stairway To Heaven. Also, any AC/DC solos, Black Sabbath, Deep Purple, ZZ Top, Metallica... listen to their music, pick a solo that makes you go \m/ and learn it!