OK here goes. BTW I should add I have the Platinum Trophies for GTS, PCARS1&2, Dirt Rally and AC and only drive using a wheel so I do know these games fairly well ;-)
Assetto Corsa is #1 for driveabilty, realism, and driving viewing position and don't forget the clutch and shifter actually works properly e.g you can actually ride the clutch. The game lacks cars and tracks and the ability to put modern tyres on old cars. The tuning is realistic too e.g you cant adjust toe or camber on a rear beam axle car. Are you reading this Kaz? ;-) Another thing to praise this game for is the audio. What great sounds some of these cars make. Wow!
PCARS 2 is #2 especially if you like messing with weather. Great amount of cars and tracks BUT they kept messing with the tyre model as in every update they changed it and then it knocked out all you tunes! >:-( So I have no idea where it is at now as I gave up on it when GT Sport launched. This also has a shifter and clutch that actually works properly. BTW don't even get me started on their online penalty system!
GT Sport I would rate at #3. Not very realistic as it panders more to the arcade type player with a hand controller. Th smooth graphics with a PS4 Pro on a 4K TV go a long way to helping with realism.
PCARS 1 at #4 and I hated it and traded it on PCARS 2.
Dirt Rally is very interesting and is a mixed bag but I would put it at #5. Yes the shifter and clutch works in this but the tarmac/bitumen tyre model and physics are just plain wrong. The game is wholly worth while just for Pykes Peak BUT the downside is you can only drive 4 cars on it :-( The older rally cars are an absolute blast where as the modern ones are just ballistic missiles with ridiculous amounts of grip.
Drive Club. I don't really want to even talk about it as it was a fiasco from the day it was announced. I had it on pre-order for 2 years until I gave up and then Sony reneged on the free download for quite a while. So eventually I did play the free download and hated every second of it. I tried and tried to get a handle on it as the full retail version was selling very cheap but I found it just too arcady and the track scenarios were just plain fantasies.
To sum up. I like real cars, on real world tracks with real world type physics. I hate cartoon cars that are some modeller's wet dream on fantasy tracks. I also like options of what my driving perspective is. I seems every other game besides GTS can do this. I like to have my viewpoint at the base of the windscreen so I can see the bonnet in front of me (no I do not want to sit on the roof as in GTS).
What was great in AC was I could have this bonnet view and then mount an Android Tablet to my wheel base and set it with all the dash instrumentation with an app. Absolute heaven. This has been the most realistic view and immersive experience of all the games I have listed.
So why do I play GTS you ask. Simple, all my friends are on it and it has the best and most stable online plus it is the prettiest by far ;-) The graphics really are superb and it is very hard going back to these other games.