I know I've seen a lot of GT3 at Spa. I like online racing but I find that I mostly end up racing in unfamiliar cars/tracks. I think the best approach is for me to focus on time trials against ghosts on popular track/car combos. What are the most popular combinations?
What system?
On PS4, GT3 is definitely the most popular. For tracks, it's usually the F1 tracks as mentioned, plus Algarve, Bathurst, Brands Hatch, Brno, Daytona, Donnington, Dubai, Fuji, Imola, Indy, Leguna Seca, Le Mans, Bugatti, Long Beach, Road America, Sakito, Sonoma, Sugo, Watkins Glen, Zhuhai, Zolder.
Start with the F1 tracks like Monza, Silverstone, Redbull Ring, Spa, Catalunya, COTA, along with Leguna Seca, Donnington, Imola, Watkins Glen...that should give you a good base to start from.
Other classes that are popular at the Formula and LMP cars. I don't do them much, but they're usually on the big F1 tracks, or the Indycar tracks in America.
I've noticed the TC class is getting more common. These cars go well on the big tracks, but also on many of the smaller national sized tracks.
For Vintage stuff, Group A and Group 5, maybe Group C, are the most common. You see these lobbies use lots of tracks, but I find it's pretty common to see them use the classic tracks, or at least tracks which have very unaltered layouts, like Watkins Glen or Road America.