Ignoring off series games, this is my order :
7) GT1 : great game obviously, the start of many things and a definitive game changer. But in retrospective, it wasn't fully matured yet, considering how much GT2 had evolved from it in such a short time. Also Initial D brake drifting physics.
6) GT3 : the switch from PS1 to PS2 era was massive in many regards, but so many content and features were lost. It felt a bit empty.
5) GTS : This is a weird one. The game changed a lot since it came out, and it showed great potential, but it hasn't been exploited and the game went backwards in many areas. The very first iteration of the physics lacked stupid amounts of traction, but that was fixed relatively quickly and 2nd or 3rd iteration was actually great. Really, those were really good physics, it felt great. Even the old classic "reversed" camber effect issue was finally fixed ! (Literally one of the first things I've experimented with in the game). That was in 2018 and that's when the game was in its best state (this also includes sport mode and FIAs). Since them, some things have improved, but more things got worse. The physics got dumbed down and came back to GT5 style understeer - actually probably worse - and virtually unlimited traction. While this has been dialed back a bit, it's still nowhere near as good as it was before, and also, all the setup "glitches" dating back from GT5 at least for some came back (like the camber thing). Other bugs still present in GTS can be traced back to GT5 as well. But the slipstream update that made it twice as strong takes the cake of being the most stupid one - they've reverted it back but it took them an absurd amount of time.
This game is extremely frustrating in how it's been handled during its life cycle, core game mechanics and Sport Mode included. Some meta knowledge here is influencing me, but I can't help seeing what it could be and what we're missing because of bad decisions / communication on Polyphony's side. Some features everyone would love are literally in the game but locked away from us.
The livery editor though is an absolute masterpiece and adds a lot to the experience. Only a couple minor tweaks would make it perfect (bring back GT5/6 paint chips in addition to the palette and special colors / individual paint type setting for stickers as well as allowing paint chips applied to them / group layers into folders / custom windshield banners and allow window stickers in some places / stop making wheel choices for some cars completely stupid).
To sum up, I hate it at least as much as I love it. It's a shame because GTS definitely has the potential deep inside to be on the podium of this list. Everyone who doesn't care much for the solo stuff and experienced 2018 GTS will tell you they miss those days.
4) GT4 : very polished game, nice features, and the first online things ever happening in Gran Turismo (through unofficial methods to run LAN remotely) - I didn't get to experience it first hand, but the tandem drifting videos from team FBI were inspiring back then. Great OST as well - I mean, they all have nice OSTs, but this one I think was the most complete regarding race music. The photomode was quite nice as well, iirc more realistic than the PS3 version of that mode. I started testing road cars on Trial Mountain, but stopped at like 200.
3) GT5 : first game of the main series than could be played online against real people, still has the best online features and modes we ever had in Gran Turismo (who doesn't miss shuffle races ?). In retrospective, the physics were very rigid, but they were also really fun to drift. I've made good friends on this game, got accidentally pushed into competitive racing, and done tons of drifting then. We were having lobbies daily at the time, great memories. Made me stop caring about solo career. The standard cars thing was weird at first, but in the end, we were glad to have the content and have fun with it. The game would have been worse without it. As a solo game, it would have been under GT4, but the online section makes the career not as important as it was for previous games.
2) GT6 : yes, it's more like a GT5.5, probably could have been a spec 3.0 update ? Some online features being cut was a big con, but communities were great, but more importantly, THE PHYSICS : they've been such a massive over GT5 "solid piece of wood struts" physics, and in retrospective, I think GT6 physics were actually better than current GTS physics for road cars. I've been thoroughly impressed by the feeling of some cars I've driven hard IRL and the steering feedback some of these had was spot on in some situations. I've had the FFB react exactly like what I experienced in real life in a Grande Punto Abarth, accelerating in 2nd gear out of a bend, and same steering feeling on corner entries in a Celica T23 for instance. I remember chatting with the Lacombe brothers and their crew and they were thinking road car physics in GT6 were slightly above Assetto in some regards (but race car physics were quite worse). Sadly, the intensity online fell down as the active community decreased, since people were waiting for a PS4 game and opted not to buy in disappointment (and it seems like Sony might not have learned from this). But objectively, those physics had me craving to drive, so I did test all the road cars in the game on Trial Mountain and made quite an extensive topic about it on here. So to me, it deserves being above GT5. Also, the "pirate" version of the track editor allowed us to produce great tracks.
1) GT2 : this game was HUGE. Absolutely insane. Very mature and polished as well for a PS1 game. Such unique cars were available, the racing transformations, the OST, but what I regret the most from this game : the circuits. Polyphony Digital are great at designing original tracks, and GT2 brought absolute gems which sadly were never featured again in the series. Red Rock, Grindelwald, the night version of Rome... I'd kill for them to return (although I still want Trial Mountain, Grand Valley, Deep Forest and SSR5 back more). It's the most complete and well rounded game of the series, relatively to the technologies available at release.
Now for GT7, I have no idea where it will fit there, because it could very well go at the bottom as much as it could go to the top. What it would need to be at the top for me :
- improved physics : mainly body and suspension elements not clipping through each other, bug fixes, stop using the dumb default 0.60 rear toe setting, more advanced tyre heating model, braking behavior without ABS.
- MORE GRID SLOTS !
- GT5 online lobby features with GT6 community tools / unlock what's locked but exists in the dev - world tour version of the game
- MORE GRID SLOTS !
- Allow people to download the livery template so they can edit it themselves if the author allows it.
- MORE GRID SLOTS !
- Dynamic weather / time of day.
- MORE GRID SLOTS !
- Better servers / ditching P2P.
- MORE GRID SLOTS !
- Classic original tracks comeback, with their layout preserved as much as possible.
- MORE GRID SLOTS !
- 120 FPS (don't need more).
- MORE GRID SLOTS !
- Content (but that's a given since existing assets are 8k ready).
- MORE GRID SLOTS !
- PS1/2 era style license tests (more extensive and more challenging - introduce a platinum medal if needed. I think that's important for solo content focused players and useful for newcomers to get the basics of driving right).
- And the most important one : More. Grid. Slots. IN LOBBIES AS WELL.
What would put it last :
- not having more grid slots in online lobbies (instant deal breaker).
- PS4 release (it would CPU limit the game's ability to run many of the improvements it needs that are tied to CPU power and not GPU power).
- Existing features being cut for no reason or dumbed down. Again.
- Understeer physics v4.
- Spaghetti code turns into more spaghetti code.