Soundtracks wise (original soundtracks), GT hands down has the best soundtrack. Here's what I can say about the soundtracks from 1-6:
GT1: This is where it all began. Nissan, Mitsubishi, and Honda Dealer theme (Japanese) were the best. Honorable mention goes to Subaru and Toyota Dealer theme.
GT2: Ah yes. They improved the menu songs from the Japanese version and made it available on all versions. Favorite theme would have to be the Garage and West City theme.
GT3: This is where they made the menu themes more GT3ish and made it feel like it's made for GT3. The dealer and race selection theme is my favorite. Oh and this is where Daiki Kasho made his debut. Gotta give him credit for remixing Are You Gonna Go My Way.
GT4: The songs are back from GT3 but made it GT4ish. Some are slow but then again, it feels relaxing and looking at car dealers made it even better with the songs. Oh and once again, Daiki Kasho is back and decided to collaborate with Dakota Star. My favorites are Break Down, It's All About You, and What To Believe.
Note: I wonder when will Daiki Kasho start collaborating with a female vocalist because it's been like GT4 that we had songs with a female vocalist.
GT5/GT5P: This is where Kaz' interest with lounge, elevator music started to appear with most of the menu music having lounge, jazz, and elevator music. GT5 and GT5P also led Daiki Kasho into prominence because of his works with Jonathan Underdown and the songs SURVIV3, 5OUL ON D!SPLAY, etc.
GT6: Same as GT5 but added more lounge music. Also I'm glad they decided to add the older Daiki Kasho songs to allow people to revisit his classics.
Overall, it evolved well.