There have been many tin-top categories over the years, but the one that appears to be by far the most prominent worldwide at the moment is GT3.
In addition to there being separate international sprint and endurance series for them (for which the Bathurst 12 Hour is normally the endurance season opener), there are also multiple domestic championships in Europe, the Americas and Australasia that centre around them exclusively or at least include them (whether mixed with the feeder GT4 class or other local classes).
The cars themselves have even helped others survive. In Germany for instance, from the 2021 season DTM (Deutsche Tourenwagen Masters; or German touring car championship) has adopted GT3-based cars after they found themselves down to the support of just one manufacturer at the end of 2020.
The Germans also have the GT Masters with cars based on the GT3 rules. Plus the SP9 class, which for several years has produced the overall winner of the 24h Rennen Nurburgring (24 hour race of the Nurburgring), is also based on GT3 (with some minor changes to vehicle specification).
Looking at GT3 itself though, despite restrictions and almost no Japanese involvement, the 2021 24 Hours of Spa was so big it had 58 entries, entirely made up of GT3s.
There are multiple reasons to explain why GT3 is popular. Here are just a few.
One is the balance of performance (BOP) equation that seems to get things pretty much right in terms of ensuring no one model of vehicle is always at the front.
The cars are rather different underneath with some front-engined, some mid-engined and Porsche still using the rear-engine layout. All of their heights, lengths and widths also differ, as do their total weights and power curves, and yet even when one has a handling advantage at a particular track, or another has a straight line advantage at a different circuit, the times are still close enough that the grids and race results remain mixed.
That leads us to another reason for its appeal, and that is the variety of cars. Lots of manufacturers based in a number of different countries with a 2-door GT car on their showroom floors to promote, have produced a GT3 vehicle that is at least based on it (and not just in looks but in several cases also their engines).
Manufacturer involvement doesn't go far past that either, operating at most by way of supporting satellite teams with technical assistance and possibly lending them a contracted professional driver or two, so private entries can still buy a car, be good at what they do and compete on equal terms. Things like limited allocation of a control tyre also helps keep things pretty level.
Another basis of appeal for GT3 is the classes they have based on driver professionalism (and experience) rather than just the cars themselves. There's Pro for just professional drivers, Pro-Am for pairings with one professional and one amateur, and Am for just the amateur drivers.
Not only does this open things up for more competitors to have a go, it's normal in Pro-Am combinations for the pro driver to develop the amateur into a quicker driver, and to help the team to develop the car setup as well. That applies to short events too. Even the GT3 sprint races that go for just one hour will have a compulsory driver change between the 25 and 35 minute mark.
Possibly the biggest reason GT3 has so much prominence worldwide is all the international and many of the domestic race broadcasts are live streamed (and a replay uploaded straight after) on YouTube in more than one language (English included) with no regional restrictions in place. More access means more potential fans and more exposure for your sponsors. Seems obvious I guess.
Also in terms of public accessibility there are multiple games that include some GT3 cars as official content. The one widely regarded as the most convincing GT3 sim (and I agree) is Assetto Corsa Competizione, which is actually dedicated to them (and now offers GT4 as an option as well) being that it is the sim officially licenced by the SRO Motorsports Group who conduct all the international and several domestic GT3 series.
