I still have several more of the modern-ish sims that I'd love to get to, but I can only talk about the games I have already played a lot. I'm not a fan of directly ranking things and don't think it means very much beyond "all these were fantastic" but I narrowed it to 10 of the ones that still stay with me years later... I think I can make a top 3 at least.
#3 Gran Turismo 5 --
Tough to narrow GT down to just one title. GT1 blew my mind for how big and realistic it was, literally changed the world for me. GT2 made GT1 feel like the worlds best demo, better at just about everything. GT3 probably was the most fun I had in career for the series. GT4 was also great but some things started to bother me like the CATASTROPHIC understeer almost every car would get stuck in sometimes and the AI just shoving you off the track the whole way down a straight. But it introduced me to the Nordschleife and again changed my world forever so that makes up for any downsides.
GT5 I have to put at the top of the GT games overall because it was much more enjoyable to drive in, had a lot of advancements and a really fun career mode(at least until the update that made the AI completely stop racing when near you). The dynamic time of day and weather made the endurance races(and your own races) so much more engaging. Somehow clearing all of the endurance races without ever winning by more than 10 seconds is an all-time achievement for me. And the online play was so much fun, whether Ring running, Shuffle racing, or just doing some general tracks... I'll always remember my GT5 online days. Sadly despite some very nice new cars added, and as much fun as driving drunk on the moon was, GT6 was a huge letdown in almost every way... lame short career, no endurance at all, AI racing essentially removed entirely as you couldn't even use them as a pace-setter, and for some reason online never really grabbed me either. Didn't play any of the newer titles. So GT5 will stay the pinnacle of Gran Turismo for me.
#2 Project CARS 2 --
I always liked to consider the first 2 Project CARS games "Gran Turismo for racing fans." An endless variety of things to do but with all focus on proper racing with sessions and generally evenly matched grids and AI that at least vaguely knows how to drive. GT was all about the cars and their history and style, PC was all about the racing itself where GT had always fallen short. But it still didn't feel to me like things were overly complicated and difficult to get started in like for example rFactor. The physics were a bit more serious but stepping up from GT to a full sim is a much smaller step than starting GT if you're used to the more arcade titles.
Again the sequel gets the nod because while the first has a few advantages, PC2 is just better and more complete in so many areas. Both PCars games had great career modes if you wanted to get lost in them, but of course you could get infinte enjoyment from just running single races. Sadly no custom championships and perhaps sadder still I never played either online. Between the two games I probably spent more time playing than anything outside of the GT series or rFactor. I'm still not satisfied I'm "done" with PC2 but currently on to other things. I'm told there's a third title but I'll probably never play it.
#1 rFactor --
Of all the racing games I've played, rFactor has to be about as close to the perfect game for me as I've seen. The base game only had limited content but there were limitless easily applied mods, and everything imaginable was so easily customizable by anyone with even a tiny bit of "tinkering" ability. I was able to adjust how sensitive cars were to damage, change the way headlights looked to make them more realistic, tweak where mirrors were aiming, create my own cars with their own liveries, even adjust cars' performance. Not to mention customizing literally everything about the races and championships; length, time of day, time of year, championship schedules and points schemes, cars allowed(even editing the mix of cars/classes in a championship if the random ones wouldn't cooperate). Everything about the game could be just how I wanted it, even things like how long debris/accidents stayed on track or how long I wanted for recon laps and gridding before endurance races.
It wasn't perfect, some things just didn't work well, and sometimes the ease of fixing things was necessary because you HAD to fix things modders set up wrong or just plain didn't bother with. But the endless amount of things to do and how close you could get them to exactly the way you wanted them makes rFactor pretty tough to top overall of the games I've played. I never played it online so can't comment. The AI had some issues(as they all do) but were a lot better than some people said once you learned how they behaved... and even a lot about how they drove could be customized, both their general behavior and the lines they took on track.
-- The rest are in no particular order --
Ferrari Formula 1(PC-1989) --
The driving in this ancient game may be crusty as hell(and I did it with 4 colors and awful sounds) but this game engaged me at the time like few ever have. It had everything about running a full-time championship effort. Testing, testing and more testing... full IRL race sessions... tweaking tons of setup options to get your car how you need it... balancing fuel consumption and car performance... even engine life over multiple races. I had endless hours of fun and it really developed my love of long races and simulation racing in general. I didn't have the computer to run the more famous and probably better Grand Prix games available at the time so this one was where sims started for me. There was a completely different arcadey console title by the same name many years later that was fun but not any resemblance to what the original did.
Road & Track Presents: The Need for Speed --
If I played them over again now I might pick High Stakes over this as a game, but this was the first one I played and the only game I owned for several weeks when I first bought my Sega Saturn so it will always have a special spot. So many great moments on some quite fun tracks, great cars that all felt different, a Supra that actually turned and even a cockpit view. I kept playing it for years off and on and I still feel it has a certain charm none of the others do.
NFS Underground 2 --
Not sure I wanted to put two NFS games on my list but this one was a good bit different than the early entries, before many of its features were combined into staples of the main series later. Both Undergrounds were fun and both really shined when played online which is why I have to include it. I think the first was the first time I ever raced online. The 2nd one just felt bigger and better in nearly every way than the first, plus the open world and better tuning options set it apart for me. I played Underground 2 for what seemed like thousands of hours in online lobbies.
TOCA Race Driver 3 --
This one feels a bit like a precursor to the Project CARS theme. I discovered it a bit late in my PS2 days and instantly fell in love. The physics maybe weren't quite as good as later titles but for immersion and racing simulation it was very tough to top at the time(particularly on consoles). And you could simulate any kind of racing you wanted, from F1 to multiclass sportscars to V8 supercars to rally and even some crazy things like monster trucks. It had a fun "quick career" that was still huge and took you through a cross-section of many of the available series, and an absolutely massive sandbox career where you could run anything you wanted with most any rules you wanted up to full-length races. I finished the quick one but never finished the sandbox career and still kind of regret that I left it just before I'd have completed my first ever full Bathurst 1,000(I'm actually not sure I've ever run one).
Race Driver GRID --
If Project CARS is GT for racing fans then I think the original GRID probably counts as NFS for racing fans. It was a little bit more realistic(I mean, you could lock up the brakes!) but was still very simple and stayed on the arcade style of ease-of-handling and excitement over realism(240mph Daytona Prototypes anyone?). But it was focused on professional racing instead of tuning up street cars. A variety of events and a quite immersive career gave me a ton of hours of fun. I eventually learned that driving with a controller was actually faster than a wheel but I still made it through the career with a wheel so it was still plenty fun and useable. I was one of the few who loved Grid Autosport too(and one of the 5% who used cockpit view in the first game) but I never got far enough through it to say it was better -- my PS3 hard drive started dying while playing it, it was the first game to become unplayable when that happened, and I never got back to it once I replaced the HDD.
Tokyo Extreme Racer 3 --
The TXR series is kind of an odd one, somewhat wonky floaty physics and a very unique approach to racing in general. I first played Zero and got quite far after getting used to it but TXR3 was the one that really caught me. Having different cities to run the highways at helped a lot to stop it from getting too monotonous, and driving with a wheel did a lot to settle the awkwardness of the handling model(I don't know if it worked in the earlier game but I never tried). From just cruising the highways to racing random people to progressing through the "story," there was a lot of fun to be had even if you got so sick of passing that same truck over and over. I'm not sure how technically good it is but it will always be one I remember fondly.
iRacing --
My current obsession finally convinced me to buy in a couple months ago. It's as expensive as everyone says, lacks a lot of what seems like common sense QoL features, has a really weird setup for pre-race sessions, and series participation can vary from thousands of people every race to half a dozen depending on what you want to race. But it all somehow works, and the driving is great fun. It's biggest feature of course is the community, if you want full sim driving with
mostly full fields and constantly being able to find races iRacing is great. Sure there's a few jerks but there always are and I'm sure iRacing's paywalls and rating systems do a lot to mitigate that. Most people are trying to race respectfully, just some of us are a bit better at it than the rest of us. There are some options to race by yourself as well when you need a break from other people, and the AI that they've added do for the most part seem to work rather well for running your own races. Just be prepared to spend and keep spending.
I'm not sure that the amount of time I've spent on it [yet] justifies its place on an all-time list, but this game very quickly resurrected my interest in online racing. Being able to do what you want when you want with the exact structure you want is a very important thing for me and only AI can do that, but the thrill of a good race online(like the one I just had with 3 of us nose-to-tail struggling over the top 3 positions for 35 minutes straight) is something that you just can't get from anything else.