Whichever car you want to use online eventually.
Whats the point in learning the track in a car you probably won't use again just because its easier? May as well skip a step and just use the car you want.
But, my recommendations would be either:
For FR cars - Jaguar XK beacuse its very easy to drive but is tail happy, it helps learning where to put the throttle down for certain corners because when it starts sliding it doesn't bite your head off.
For 4WD cars - any will do.
For FF cars - Integra Type R because if you have a brain you will use this a lot

For MR cars - Honda NSX because if you learn a track with this car you will be fast indeed.
I think a very key thing about choice of car is: if you don't learn the track's little details, you may find some cars uncontrollable, for example, if you learnt Fuji in a 4WD car, you wouldn't encounter the problems the off camber corners can cause in FR and MR cars. Same goes for hairpins, you need much smoother throttle application and sometimes a different line to help put the power down in a, for example, Dodge Viper than you do for a Subaru Impreza.