I think I've played at its moment the entire franchise, except for the "prologues" versions, and only in single player modes because online modes never worked good in my region, they were too full of issues.
These are my Top 3:
1. GT 6 : The one I enjoyed the most, with very good handling, lots of cars and lots of tracks and events.
2. GT 7 : The only game that I play at the moment on my poor PS5, it started good and stylish, but begins to feel empty when passed some weeks. At least it has some weekly events to keep you playing some more.
3. GT 4 : An incredible Gran Turismo for its time, running on a PS2 as if it was an early PS3 game. Only surpassed by 1st and 2nd place because of the use of analogue triggers to accelerate/brake.
Honorable mentions to Gran Turismo for PSP, Gran Turismo 2 and a nostalgic point to the japanese intro of the first Gran Turismo, with "Moon over the castle" as theme song.