An extension to the OP. What cars were the quickest and longest for everyone to master? Not just in a race, but have got you to a mission check point quicker than others or out of trouble against a focused player in free roam as well.
For me that adder was the quickest due to its inherent stability and grip. Speed doesn't really phase it either. There are nippier cars, but few as good all round.
The hardest two were the comet (twitchy, especially over undulations, but given measure is very quick). Off throttle it is very controllable and as long as the rear isn't allowed to bounce too much, your sweet.
The other is actually the rat loader. Not the quickest and stubborn as hell to turn. The brakes are lifeless due in part to the front heavy handling. When accelerating the back is light and prone to spin. If you can learn to drive the loader as quick as most can drive other cars then you can drive pretty much any car in my opinion.
For me, the Comet was child's play.
The hardest was the Surano (Sports).
It is very unstable and has a ton of lift-off over-steer, even more than the Comet.