Was very curious to see what people have to say about each class and what they think is the best.
I think this Forza is the most even in classes and it really depends on tunes and tracks, but still some cars are glitched. For example a 650kw Nissan r35 GTR can hit 450 km/h or 280 mph which is absolutely impossible. It took a 2000+ bhp Ford GT to hit that velocity at texas mile, but to the subject. I almost only run A class and I must say many of the musclecars do really good and ofc the lightweaight rockets, like xbow and r500. Still I do not care the least about which cars are faster. I drive the cars I like, and do well 
Look at the leaderboard
Those don’t always show a complete story but is a good starting point. Some cars aren’t represented well because they are a little slower than another car. In both cases I’m talking about cars hitting top 50/100s.
Basically if you’re a good driver, almost every car in the game is good somewhere.
It’s just a matter of being curious and trying new stuff.
Sorry I went with short answer.
What Does OP mean by Best???
LB car put down the fastest time.
LB cars with less aggressive tunes usually make good lobby cars.
Most cars can be competitive as long as you can build them for the track needs.