For me, AC is the vintage sim of my dreams. Actually, with tracks like Fonteny, it's better than my dreams.
Basically, what immerses different people is different. There's no right answer to "this is the most immersive detail" because it's up to the individual, in the same way as any subjective thing. For some it's fidelity in a tyre model nothing else matters, for some it's the realism of graphics. Nothing wrong with either.
But for me, one of the ones that gets me is the setting and location for the tracks as well as the fields. There are several games with believable vintage content but none with the quality and breadth of tracks that, in particular FatAlfie has produced.
For example, just off the top of my head, we have pre-80s versions of:
- FatAlfie (Fonteny, Thomson, Deutchlandring, Bremgarten, Battenburg ring, Feldbergring)
- Lilski (Bridgehampton, Riverside)
- Sergio Loro (Stardust, Watkins Glen, Zandvoort, Rouen, Hockenheim, Norisring, Sudschleife and many more)
- Reboot Team (Spa, Monaco)
- Thiago Lima (Longford)
- Terra21 et al (Le Mans)
- Zwiss (Zeltweg)
- WilliamTRiker (Fuji, Nurburgring)
- Tyrone (Sebring)
- Italotracks (Napoli)
- THL (Russelsheim, Gelsenkirchen)
- Thrashy (Lake Garnett)
- GaryUK (Bryar)
- Jim Lloyd (Donington)
- Abulzz (Targa Effing Florio!)
And sorry to look the gift horses in the mouth, but so many more in the pipeline! Such as Zwiss' Crystal Palace, Italotracks' Pescara, THL's Tulln and whatever Sergio and FatAlfie may surprise us with.
And then to match those , we have some glorious vintage cars and car sets. Even aside from some of the great Kunos content, we have amazing vintage cars from the likes of Pessio/, Stereo/aphidgod (seriously, that Chinook!) and then the superb ACL packs (particular shout out for the 60s GT cars in the GTC pack) from Bazza/DrDoomsLab and their supporting cast. And there are too many to mention here, so sorry for only selecting a few.
So TLDR, there's nowhere else that would let me run a realistic-feeling, period correct grid of GTs and sportscars, with appropriate liveries and drivers, on period tracks.
And, for me, that's immersion I've never had before.
Sorry, I rambled a bit there.