How do I know which cars will perform best? Is there just an overall best car for all tracks, or does each track has a car thats only best for it?

that's for you to figure out, hence the competition to be the fastest on speedruns.

For time trials, the MINI Cooper SX1 and SUBARU WRX STI RX SUPERCAR are almost always what you want. To figure out which car to choose, go to the in-game leaderboard, and check the top 20, if there's more of one car, use that.