I'm torn between most of them. GT2 (all U.S.), 3, 4, Prologue, 5 and 6. They all put me in the mood to tear around a course as hard as I can, and they were all tastefully done. I prefer the daiki kasho music the most, as well as the epic "Moon Over The Castle."
I was inspired to dig into my old GT4 folder, and I have a small treasure of videos from the pre-release days of that last hurrah on PS2. One or two even have early versions of "What To Believe," and I actually prefer it. I think SONY should offer it as an alternate track in GT7, or even Sport. They bring back such intense feelings of anticipation for the best Gran Turismo to date, and such a HUGE game. I have to watch one more time.