I've been using a lot of different cars to run Le Mans, even trying to tune some myself, and some cars are just good to drive.
If you're looking for a Gr. 4 car, I would recommend the 86, that thing is a rocket, and did 4 laps on a tank for me, not sure how it would do on AT.
I detuned the Suzuki Gr. 3 to 700 PP, with stock settings, and it's very good too. The BMW M6 (Sprint version I think) is a bit slower, especially on the higher gears (it takes a lot to change gears on AT), but it's nice to drive too.
Now for road cars, the Diablo GT, the F40 and the Ford GT are my favorites. Z4-Swapped M3 and Supra-Swapped BRZ S are on rails. Some other cars I used and I didn't see mentioned: NSX (don't remember which, but it's one of the older ones), AMG Black Series (stock), Hellcat and R8 are some of the nicest cars I've taken to Le Mans.
Bonus: if you're using MT, consider learning to drive the DP-100, it's the fastest car I've used so far, as in, sub 4:00 on fuel 6.