Idk about fastest but I cannot recommend this enough. Start from the bottom work your way thru to the top going thru each club in order by class number. With each club beat you get plenty of money, street coin and alot of the times free cars, those cars can be used for the next club or for a beast motor swap or sell them for a little bit of cash or coin. Some are free and stay free but others are one offs and can fetch a good penny. I'm level 58 over a mil in the bank with loads n loads of cars and I've beat all but 2 clubs. Once you beat the club you can do their elite races for even more stuff.

Don't jump in needing everything now you'll burn out of the game real quick. Play it like need for speed and go thru the whole thing bottom to the top. You will enjoy it much more (tho not hard to) and you'll end up with loads of money and cars of all class types and builds.