One Winged Angel.
Aerith's theme music (piano version, like when she dies)
Battle music + Chocobo.

For some really bizarre reason when Britney's "Hit me baby one more time" came out I always used to associate that with the music from Midgar's Mako reactor.

Of course, if (when) they remake FFVII on for the PS3 they should swap the music with mixes of FFVII music from the OCRemix website. Because OCRemix improves on perfection.