Gr.3 is definitely my strongest class, as it is the flagship category of the Manufacturer Series, plus the fact that the drivetrains are restricted to RWDs only without any fancy hybrid system that some Gr.1 cars have. It's also helped that most liveries that I liked to run (looking at Itasha liveries, mind you) are for Gr.3 cars, especially if they are Sport Mode compatible and have Center Lock wheels. I do also like Gr.2, but you probably should stay away from the 2008 cars in order to be competitive; they are there as just another racing livery canvas to work on.
I would say that N-class is probably my weakest point, but even then I'm still strong at them, provided that the cars are grippy enough and not understeering too much, especially high-speed tracks like Tokyo South during an N500 FIA race when the 911 GT3 RS 991 shifts its weight too far to the rear, and then we got the X-Bow which is just too light for that kind of power without downforce. Shift times being a factor also didn't help (Group C cars in Gr.1 also suffer from this due to their Racing-spec Manuals being, well, H-pattern Manuals!)