I've done probably 8-900 online races through online adventure, with about a ~68% winrate, so ill chip in. I used a lot of cars, but these are the ones I ended up using:

S2 class:

Asphalt: Lamborghini Huracan HE

Cross Country: Jeep Trailcat

S1 class:

Asphalt: Ferrari F12 or Mclaren F1 GTR

Cross country: Jeep Trailcat or #43 pro truck (i think thats what its called)

A class:

Asphalt: Ferrari 250 GTO (this is pricey though, I also had some success with the merc E63, Shelby daytona (also expensive), and the various lancer evos/imprezas

Cross country: I usually used a Ford Raptor HE but you can probably get away with using your asphalt car

B class: anything really goes here, I found success with a lot of cars.

Asphalt: Ferrari 250 California (expensive), shelby cobra

Cross country: same as asphalt here and below, as the proper off roaders dont have the raw power needed to be competitive

C and D class: this is where it gets weird. I usually used a really old car (mercury coupe and ford coupe) and put the most power I could into it, and didnt touch the tyres. So you ended up with an insane car on the straights at the expense of cornering. Depends how badly you wanted to win I suppose

Some basic tips would be to ALWAYS make sure youur car is at the very top of your class. So have a performance rating of 800 if youre in A class, 900 for S1 class, etc. Any points below that are free advantages for your opponents. The only exception is the S2 Jeep Trailcat, i think that maxes out at 903 instead of 998. But frankly that car is busted on cross country tracks

I found that most cars needed a few upgrades to be competitive. At the very least, upgrade the brakes, transmission/clutch and flywheel. Racing cars stock doesnt really work imo

Also, its not so much what you drive as how you drive it. Make sure youre braking early, taking the racing line, etc etc. online races are kinda chaotic because everyone likes to ram you, and sadly theres not a lot you can do about it other than doing your best to avoid it