Awesome Music: PGR had many beautiful songs. Ranging from fist-pumping Electronic Music, skull-crushing Industrial, head-bang friendly Heavy Metal, to moving rock music, and EVEN lovely classical music, this is musical heaven.
Bonus points for 3: it detains the Guinness World Record for "Largest Licensed Racing Game Soundtrack", with 90 songs in the playlist!
Best Level Ever: Every Major tournament in the career mode of PGR4. The first is a time-attack based tournament, in which the 8 racers who post the fastest times on the 3 rounds will face each other in a final last-man-standing race; the second is multiple-bracket tournament, with the final being an Ultimate Showdown of Ultimate Destiny between two racers; the third and last one is like the second, but taken Up to Eleven: the final round is a race between 4 racers instead of just two.
Game-Breaker: The Ferrari F50 GT in 3. Wanna know why it has been referenced so much in the main page? It is the most expensive car in the game (it costs 1.3 million credits!) for a reason. Very fast, lightning-quick acceleration, ungodly handling and grip, and it drifts well enough to give you a ton of Kudos!
Unfortunately, it has been very heavily nerfed in 4, seemingly in favor of the Caparo T1 and the SSC Ultimate Aero. And it still is very fast and controllable in that game.
At the beginning of PGR3, there is another example of Game-Breaker. See that Ultima GTR? Yes, that cute, little speedster? I have two words to tell you: BUY IT. It's substantially a toned-down F50 GT.
Yet ANOTHER Game-Breaker example in 3 is the final concept car unlocked in the game: the RUF Supercar Concept. It doesn't go nearly as fast nor handles as good as the Ferrari F50 GT, but has a much more controllable drifting. In fact, this car is sometimes used for world records, alongside the F50 GT and the Ultima GTR.
Killer App: 3 was definitely one of them at the launch of the Xbox 360. 4 could've been one of them, if it wasn't for Forza Motorsport 2, which has exited at around the same time as PGR4 and stripped the killer app title from the game.