I mostly use deep forest, but after that online event with GTRs I thought there are a lot of prolonged turns in SSR5 that really expose the car's characteristics.
An ideal track would have a nice mix of high speed as well as low speed corners.
Some cars behave quite differently in the two, mainly cars that rely on downforce.
Me personally, I use grand valley speedway. A friend of mine uses Autumn ring.
Lots of people on the tuning forums seems to use Nurburgring, but I'm not confident enough to go there to compare lap times for cars. That's still a driver's race for me.
Depends on the kind of track you are going to race. There's no ideal tune for all tracks, or even most tracks. Maybe like 1/5th of the tracks per tune type. Ideally you should be tuning for each track though.