I only drive a few classes often enough to give a favourite or where I can judge how competitive they are.
In D class my favourite car has to be the BMW 2002 Turbo, I'm using a tune (will check later who from) which can hold its own on any track apart from maybe Le Mans/Indy Oval. The handling of this car is incredible and unless you're actively trying to it's quite hard to slide.
C class I mostly use the 1970 Camaro, it has good acceleration and speed, the handling isn't perfect but still good enough so you don't get completely left behind.
Jumping to R class I use two cars (both DLC), the BMW M3 Race Car which I find is good for higher speed tracks, I've tried tuning it myself and I can't get it to grip enough out of slow corners so when a track full of slow-medium speed corners comes up I use the Mazda USF2000 as it's easy to chuck into them and accelerate out with minimum wheelspin. In overall pace I don't think these two are competitive with the top cars in R class but with a good driver behind the wheel you can certainly give them a run for their money.