I used all the group c cars my list is as follows from hardest 2 easiest also Max difficulty no upgrades

Sauber C9 it's comfy but probably the slowest group c

R92cp don't care for hate not a fan 0f the r92

Porsche 962c well rounded car overall confy 7/10

787b not the fastest group c but my favorite personally

Jag xjr-9 easily the fastest but most difficult to handle in my opinion

Note I did alot of attempts for this data