GT1 because nostalgic feel.
GT2 US version for that cool historic theme, and My Favourite Game.
GT3, because Supra.
GT4 Japan edition, the orchestra part of Moon Over The Castle is great.
GT5 all version (including the MCR, haters gonna hate) the songs and the movie intro just fits perfectly. I like the part where the 3 historical dream cars racing in Mulsanne Straight while the MCR song just whispers on the background.
The Japan version with The rearranged version of MotC is just plain epic. Spec II with Soul on Display is awesome too.
GT6 intro movie kinda grow on me now, the first part where the drums started fading in with the camera speeds off on nurburgring and the "O..O...O..." part (lol) with the number 6 filled cars, GTR GT3 in pitstop is just friggin epic, oh yes the outro that starts with Stratos, R8 Gordini and Alpine racing with the engine sounds filling over the interlude of the song gave me goosebumps, too bad the sounds aren't like that in the game.