what is the best derelict car for each car class

Hard to say, I think it's a lot the choise of the gamer, but not one is better than each other.

I would make it this way (if you want to have one derelict per class:

Bel Air (secound choise Beetle) - Race

Beetle (secound choise Bel Air) - Offroad

240SX - Drift

Mustang (secound choise C10) - Drag

C10 (secound choise Mustang) - Runner

as far as i can see in online matchmaking,

Bel Air(not tested, but always seen in the top 1-2) and Mustang(tested the stang, and it's surprisingly more stable and has 15mph more than Lancer) for off road

Beetle for racing(technical tracs)

the other are just average, tho i liked my Fairlady in career races as it's acceleration is one of the best in the game(alongside the new NSX), but it lacks top speed

I'd use fairlady Z for all.

it is hands down my best driftcar! <3

The fair lady makes drifting super easy

The mustang I would think off road cause of the constant speed changes that it would stay consistent.

The beetle would make a great racer due to being light and fast.

The C10 is a. Tank and runner is. Always plowing into cars so it would make the most sense

And as for drag......bel air.

*Note* I only using my head to over think this. I only have the fair lady as a drifter so this may change once I get the other ones*