Without a doubt the TVR Speed 12 on S1 as RWD without aero. Challenging, rewarding and fun. Once it grips nothing can compete with its acceleration.
Second would be the Cadillac CTS-V Sedan on A as RWD without aero. Not competitive but just a brute force.
Fun cars for me have to always struggle with traction when putting the hammer down at lower speeds. There is just nothing else than RWD.
Most Driven Car: Pagani Zonda Cinque Roadster
Most succesful Car: Tuned Porsche 917k
I love my SL65 (normal version) the most. The torque monster of an engine is just amazing. And the exhaust pops… 
My personal fave car is currently the Hoonigan RS200. Sensational car.
Others I use regularly are the Ascari KZ1 (very underrated) and the McLaren 720S PO
For Skill point grinding its all about the Renault 5 FE although all the above will accrue points quite nicely.
I drive the Lamborghini Reventon FE mostly. It like the payout you get on the Goliath.
AMC (James Bond) Hornet and the Mk6 VW Golf R. The Audi TTS is up there as well.
2016 Alfa Giulia FE upgraded to 900 / S1 as standard and Forzathon Live participant. Other cars like Aston Vulcan FE or Hypercars for speed zones or danger signs. For challenges I choose and upgrade cars which I like - that can range from Ford F100 to BMW M1. I like the variety the game offers.
The skyline R34 and Audi S1
I like the 4wd Subaru’s, they are obviously good off road and are good on road and can even drift without a drift tune. They are just so versatile, i also like the off road buggy’s.
Pretty sure most time is in Hoonitruck, fine tuning my drift setup, I spend lot of time sideways. No idea what is the most successful. Ranked races tend to variate so i do drive many cars in many different races.
X-Class Agera RS is my main freeroam car for lives or PR stunts. I think I have like 42K miles on it. Fantastic car
I’ve got a lot tied for most driven because I have a full garage and I haven’t gotten through tuning and driving them all yet. I started with the fastest cars, but I also work from left to right in the garage, and sometimes right to left. Sometimes random. I’ve liked all of 'em, except the 3 wheeled cars and the Hot Wheels cars. Unless it’s a really stupid car with completely ridiculous upgrades that keeps me from enjoying its natural silliness (looking at you, P50 with the big engine taking up zero space), or toys cars or bad toy car to real world car builds … unless it’s one of those, I don’t descriminate much. Eh, not a fan of the huge trucks, either, but they aren’t immersion breaking like those others.
My favorites are the Ferrari F50 GT … my favorite real world car … and the BMW E39 M5 … because that’s been my real world ride for the better part of two decades. Love that car.
My most driven is the Lancia Delta S4.
Cars I’ve been using recently are the Dodge Demon, 20019 Porsche 911 Carrera S, and NSX-R GT.
Lamborghini Veneno, Pagani Huayra BC and the '15 Honda Civic Type R are probably my top three.
I’m also driving around a lot in classics lately, especially the Lamborghini Miura, the Lotus Elan and the Mercedes W154
Reventon FE for skill points/cash farming. download my tune shameless plug