I don´t know... Here´s the thing:
With GT1, I was impressed, since there was nothing like it in those days... Even with its flaws, it was THE game to have for car enthusiasts and gamers like me. And one thing I keep in my heart about GT1: Thanks to it, I know what a Nissan Skyline GTR, a Subaru Impreza and a Lancer Evo is... And thanks to it, you see them rolling over here in this side of the planet...
GT2 was probably a dream come true. All of the cars that you had, the imrpoved realistic sounds, were every car had a distinctive sound unlike GT1 wich had only like three sounds, plus all the events, music, wheels, tracks, Muscle cars, vintage cars, race mods, etc... I really learned alot from this game.
GT3 was pure eye candy, and basically made me buy a PS2. Then I realized that even with the shortage of content vs GT2, the game lasted for years, and it was probably one of the gratest GT´s I´ve ever played, plus the best handling physics in any GT so far.
GT4... First of all, the intro made me shead tears of joy... With this masterpiece, PD showed the whole world what The Real Driving Simulator was all about. I fell in love with the special events, the prize system, the music, the performance improvement aspect of it. Everything! Even with some little details like some wrong engine sound samples, this was THE Gran Turismo. Since the release date to now, I was still playing it, and not getting bored of it...
GT5... Again, PD not only showed how a driving game should be, but it showed the gamer comunity what incredible ARTISTS work in PD´s facility. I can literally spend hours in photo mode discovering mind blowing details in the premium cars. My personal favorite aspects are: The driving physics are better than ever! Hand to hand with GT3´s physics. The graphics are just amazing... The sound improvements deserve a big recognition, the A.I, although not perfect, finally makes a the race enjoyable... Weather effects still make me wonder if what I´m seeing is real or not... And again, this game is pure driving pleasure...
But I have to chose GT2... Simply because in those days, I was not so techy when it came to cars, and I didn´t noticed "details" like I do now. And since it was my passion, and I was a kid, I enjoyed that game in the most pure enthusiastic way possible!