dope beats, dope rhymes, a good hook.

the beats, whether looped (East Flatbush Project - Tried By 12) or not (The Pharcyde - Runnin'), should be something you can guess on Name That Tune in 3 notes or less.

the rhymes don't have to be hip-hop quotable line for line, but at least have lines that people can't forget: "i came in the door, i said it before..." (c)Rakim; "i met this girl when i was 10 years old..." (c)Com Sense.

the hook, whether sung (The Fugess - Fu-gee-la, The Roots - What They Do) or not(ATCQ - Bonita Applebaum) MUST be something people can't get outta they head.

i guess the word here is MEMORABLE. shit that's timeless. one of the three parts being a masterpiece won't cut it. each of the three must contribute for the song as a whole to be classic.