The Ford Escort RS1800, not the most well known car but I have a lot of success with it, it needs some upgrades to get it up to a D car and then you tune its gear ratios to increase its acceleration. It tops out at about 135mph the way I have it but it’s acceleration and handling more than make up for it, just try and stay in the low gears for as long as you can on the straights…and watch the live rear axle, it makes for really good fun 
Its seem the SÚV are very competitive in D Class
There isn’t one car that dominates like the Mini did in 5. D class is very competitive. It does seem to vary greatly from track to track. The usual performers are still very good. Like the Elan, Civic and Celica. There are a lot of cars that are really good that weren’t in 5 at all or they were just terrible. Forza relly seems to have chopped the legs out from under the Mini in 6 as its very good only on certain tracks, this has leveled the field overall. The Clio is excellent, so is the 92 VW and the Alfa Spider. Even the Country Squire is good on a few tracks. The trucks and suv’s are way better than they should be. Its unfair that they handle the way they do.
By far, the most versatile car is the Cosmo. Its dominate on several tracks, but its very good on EVERY track. The one guy has a #1 time with it on Prague Reverse and also has a #1 on one of the long Lemans tracks. Thats impressive range from one car, IMO. There’s a slight catch to it though. The tune and build need to be altered for each track. You’re not going to be consistently competitive with just one build. 5 to 10 hp makes a huge difference. So do ARB settings and aero. I have a ton of builds for it that range from 310hp/3.8 handling to 150hp/5.8 handling. They are all great on the tracks they were tuned for. There are better cars than the Cosmo on most tracks, but nothing comes close to it in terms of versatility. Its also really really good in the rain. The huge downside to it is that it does not like contact at all. The slightest bump from another car in a lobby can be ridiculous. In a lobby with clean drivers, its so incredibly easy to win races with. In a lobby full of wreckers, drive something else unless you want to do a lot of donuts in the grass. There are some decent tunes for it out there, but its best if you can tune it yourself.