It might be a bit of a cliché, but my pick would be Teenage Kicks by the Undertones. It's such a perfect representation of its times, there's a longing in the lyrics that I still feel every time I hear the song, and while not technically a masterpiece in terms of depth or skills, there's nothing you could add or change that would make the song any better.

Or if a partial song counts, there's of course that guitar solo by Prince around the 03:25 mark.