It's interesting to see how everyone's experiences have differed with each and every title, and how different our opinions on some titles can be.
Personally, I'll always rank GT2 as the best Gran Turismo. While it was flawed in some regards (there's still no way to get 100% completion, the rally AI was absolutely terrible in the first version of the game before they re-released it with a patch) it offered an amount of content that was unheard of at the time, and not only kept the features that made the first game an international hit, but improved upon them as well. I learned of the existence of so many cars due to GT2, stuff that was never available in North America like the Kei Cars (hello Daihatsu Midget!) and varying older Japanese cars while still being quite diverse, something that I feel has not been quite the case with the later titles. The added tracks like Apricot Hill and Midfield Raceway provided a lot of intense racing action, we had a glimpse of the Pikes Peak hillclimb, and we got Laguna Seca as an added bonus. It was fun, the soundtrack was killer, and quite simply, it's
my favorite game.
GT1 is second. It's the daddy, the one that changed the way a racing game should be, the one reason why we're all here debating which of those games is the best. It's genesis, and it's not aged well, but it doesn't matter. It was so involving, so interesting to play, and it ultimately made every racing game developer take notice. The king is dead, long live the king.
GT3 would come next, easily the best driving game of the lot, but with too few features to really make it to #1, and perhaps too much seriousness to make it to #2. The car list was more of a best of than anything, the track list was more or less the same one from GT2, but featured a lot of races to keep you entertained for a while. But the variety races sort of hinted at a direction that would become clear in GT4, something I wasn't sure about at the time but today can say really changed the way GT is played, eventually the game became long and tedious and struggled to keep me entertained. Plus, the random prize car draw was kind of annoying, especially at the end of a long championship or endurance race, where you had the chance to end up with a Miata instead of the F688/S (true story). I think I had something like 6 Gillet Vertigo at some point.
I wanted to put GT4 even further down the list but thankfully for it the title that followed managed to be even worse, so fourth it goes. It's the one I've played most, too, which might be odd considering how low it is on my list. I very much have an hate/love relationship with this one, but sadly the memories I have of it are mostly negative. On paper it looked like it would be the best game in the world, the car list was massive, the track list was impressive, and the early reports showed a multitude of events that would prove to make it last for years and years on. Except the massive car list, while diverse was not as diverse as one would have expected, with A LOT of cars that were basically the same aside from some small cosmetic details (an R34 GT-R V Spec II is exactly the same car as an R34 M-Spec Nur aside the goddamned color, folks.), and it featured some of the most useless cars ever added to a game, like the Benz Patent Wagen that couldn't climb a small hill. It became very clear this was turning into a car encyclopedia much more than a racing game, something that would later plague the series in other ways, like the photo mode, which in itself is quite interesting but always felt a bit misplaced. The physics were the worst in the franchise at this point, the sounds weren't impressive, graphically I always felt it was a step down from GT3. And then we come to the worst part, the part that makes me hate that game so very much; the actual racing. In about 85-90% of the events the AI wasn't able to deliver a suitable challenge, they were abysmally slow. But that 10-15%? Good luck. The game got hard for the sake of being hard, and I always felt the rewards never matched the effort it took. It got frustrating, too. The three 24h enduros got tiresome, they even broke my first PS2 back in the day. The fact you had to either drive them 24h straight by yourself, without being able to save in progress, or having the option to turn to B-Spec Bob to help you was an absolute joke, seeing how the B-Spec AI was absolutely terrible. It was incredibly grindy, something that would become a staple feature of GT5... But wait, we come to the piece de résistance, the one thing that nearly broke me, and broke at least 2 controllers out of sheer frustration. Mission 34 was, and still is to this day, the worst thing I've come across in a racing game. You had to be absolutely inch perfect while dicing around an AI that was absolutely oblivious to you and that you would obviously come across in the worst spots on the racetrack, in an understeering pig of a car that was only good for going straight... And if you made one tiny mistake, you had to restart the whole thing and spend 137 seconds sitting still, every time. It took me about 2-3 years to get through this thing, because I couldn't accept having a 99,6% completion average on that game, it became an obsession. And then I finally got it, and I shoved the game in a drawer and didn't play it until my mother found it and all my PS2 stuff a few months ago in a box in my childhood house's basement. Nostalgia got the best of me and I had to try it again... Nope, still won't change my mind.
GT5 is next, obviously. By the time it came out, I had ultimately jumped ship, got myself a 360 and was enjoying FM3, because 5 years is a long ass time between 2 titles, and even as a diehard fan I realised something just wasn't quite right with PD. It's the only Gran Turismo title I've not played at least 10 hours, because after 3, I absolutely could not care anymore, I could not get myself in the zone and actually enjoy it. To say it was a tedious experience would be an understatement. It failed to deliver on every aspect, from the sounds to the physics, to the massive car list that ended up being GT4 cars ported over with little to no improvement. Lazy. It was at this point I realised just how much GT had missed the mark, just how good FM3 had become (and how the following FM4 absolutely raped everyone on the racing game market, but let's not throw more fuel on the fire here) It was the end of an era for me, the series I loved and cherished since I was a teenager had lost all its appeal, and I have to say it was quite sad.
And as a result, I have not played GT6 at all, and to be honest I don't think I will bother with GTSport either, at least not from what I'm seeing so far.
This choice is a bit weird here, because, objectively, the game was quite meh in terms of overall performance and presentation. My great experience in online racing actually made me to put GT5 here, so.. I believe it's a decent reason. 
