Generally there are a lot of different GP-Style tracks out right now. From very Classic "Oldschool Proping" to very smooth custom stunt tracks. So figuring out the style of tracks you enjoy the most is the key.
There are a lot of really good other creators around aswell though. I would suggest you try out some of the listed people above and you should join some playlists and bookmark the tracks you enjoy the most.
You could also try out some of my creations if you want. Fishy Tracks and give me some feedback.
It depends on what type of tracks you enjoy the most If its GP style track than I can't help you but if it's good stunt tracks you want I'm your guy, I got about 200 top-notch stunt races spanning 2 accounts (2nd account to search on the SC for is sweetgamergirl89) that work with pretty much all vehicle classes (except for MC's).
I would suggest trying newer tracks by the people imfishy mentioned, and also highly recommend imfishys tracks.. For some great custom stunt races I would recommend anything by cyclopic or subzero, they both have well built tracks and great variety. On mobile but can link their profiles later if you want