This is usually where I would say that GT1's is the best. But really, I tend to like Gran Turismo 4's intro the most. I still remember a few times where I shed a tear because of the beautiful operatic singing followed by "Panama" by Van Halen. Even for a guy who doesn't religiously listen to rock music, I'm a fan of "Panama." I still think "Moon Over the Castle" should always be the theme for GT. No need to pander to certain audiences with half-assed or annoying songs to open up Gran Turismo. Does a different theme play when James Bond walks towards the center and shoots? So "Moon Over the Castle" or no intro at all.
My vote is for GT4's intro. However, GT3 tends to grip me the most music-wise as I'm actually a Lenny Kravitz fan. Rather hear Lenny than My Chemical Romance (GT5).