Well not being baised or anything ( )
Out of the 3 if you can only have one I would have the car modelling
Reasons:
1) its longer so therefore more hands on practice and shows every aspect from conception to final render
2) The techniques can be easily transfered to organic models for example I am currently working on a raptor using the exact techniques taught in the tutorial
3) It also covers some mental ray settings needed for the final render
4) It's mine

hope it helps