My introduction to GT was GT5 starting with Prologue. I was a PC gamer for much of the previous 30 years before switching to console for my gaming needs with a PS3.
.....so. I don't think I'm qualified to make a choice given I didn't play any of the previous GT's until GT5 although I wasn't new to simulators having played others on PC. However, I will say that GT5 was likely the better of the latest titles in that regard but that's not dismissing the advances in the future iterations.
Unfortunately though, it's a step forward in some ways but then a step back in others when talking about GT6, Sport and GT7. Actually, I skipped GT Sport all together initially because of their focus on Online. ....but then they updated the game as years passed and included a single-player element. In fact, I think the SP in Sport was better laid out than GT7's. ....and I am not a fan of the Menu Books.
Honesty, I wish they would simply create a proper career mode where I could work my way up through the ranks in various classes, seasons, an event calendar, etc. Maybe an economy to help purchase / build / repair my car(s), etc. Perhaps radio chatter from the crew chief notifying me of yellow flags, opponent positions, pit advice and so on. There was a game I remember playing that had that which escapes me. I always though it was kinda of cool.
In either case, there is a a lot I like about GT7 compared to previous titles and aspects I don't. The campaign is one of the ones I don't particularly care for and even more so, the custom race mode given that the payouts are unfairly low, there is no save feature for ones you create and so on.
My wish is that they at least come through on improving Custom because at lest I can create my own experiences like a mixed field of GT cars and so on.