First era: 1994's The Need for Speed to 2002's Hot Pursuit 2
Headless Horse, Pavlova, Heinerklingle and Sanoqoua from NFS II. Besides their perfect match with corresponding racing circuits, these tracks were clearly meant to be played while you are driving in high speeds. Some would find the rock/metal selections just as appropriate for similar or different reasons (take thesenumbers for examples).
Heck, even the tracks that last less then a minute are awesome, such as the themes in showcase videos from NFS II (like the McLaren F1, or Ford GT90).
The demonstration tracks in NFS III and High Stakes when the game is left on the menu for long enough.
The song used for the game's intro cinematic: "Going Down on It" by Hot Action Cop. If you get it, be careful though; the game version is insanely different, and worlds better, from the very dirty album version.
"Fever For The Flava", also by Hot Action Cop. Which is also much different from (and better than) the very dirty album version.
Second era: 2003's Underground to 2008's Undercover