Need for Speed: Underground is the quintesential night racing scene game that started my love (and hate) relationship with modded cars and underglow neon. It also shaped my music taste as a kid/teen, which I guess is not a bad thing either, because the music was generally quite good :)
The most important thing, however - the racing! The physics were alright at the time, the rubberbanding AI was frustrating, but not unbeatable (even at hard), but the visuals and the look of the cars made it all worthwhile :)
And who could forget the intro music when you land on the main menu...