I got the endurance pack 3 months ago and the gt3 pack yesterday.
The physics and handling of these cars is the best I've ever came across!
They are the only cars where I can control the car's rotation with both brake and throttle. Also they are nearly impossible to spin for me, while doing some powerslides because the rears lock up or because I mashed the throttle too hard happens quite frequently!
Imo if you got the money ready (gt3 is on sale right now, endurance pack not sure), go get them! Endurance pack might be better because these cars are built to use without abs so you learn brake control too!
In my opinion the best car is not the hardest one to drive. It's more about how well you can feel everything that's needed to be learnt. These two packs are the ones where I feel the most and therefore learn a lot!
Going back to ac or raceroom with the same cars (amg gt3 in this case for me) shows that I learnt a lot already. The other Sims, where I feel less regarding brake and throttle are now easier too
In the end you should get a car that can do everything without being too quick. F1 cars are too quick, a road car without camber and abs isn't the right thing too
And it should be fun so you enjoy it and don't get frustrated.
Again, endurance and gt3 pack do this absolutely perfectly!