GT6 has to be the best because of the post-launch support. It also had a ton of free features like the massive Sierra Circuit, and some great events like the Goodwood Hillclimb.
Yes, although sometimes, you don't even have to play the game at all, you can just watch it first on Youtube.
I'll be very honest... When it comes to video games and it's about Gran Turismo, I honestly can't pick which series is best, because I just like every single one. Of course I have my preferences, but it's not as close to the point that I can choose just a specific title.