The Score
Dust to Dust
From the pulse-pounding introduction (appropriate since the opening credits are inside our main character's body) to the IKEA-shopping scene background tune that reminds us of elevator music to the fast-paced chase as our main character runs from himself (literally), the tone for the film is set by the music.
Just as David Fincher worked in music videos before making the leap to the big screen, the Dust Brothers worked with the hottest artists of the 90s, like Tone-Loc, the Beastie Boys, Beck, and even Hanson). What, "MMMBop" wasn't appropriate for Fight Club?