Generally. low down front three quarters with a tiny bit of lock on gets my vote, the lower you get the camera the more imposing it makes the car look.
Though it depends on the car and location, if you want more of the scenery in, higher works better, think i'll always like this high rear three quarters one i took of the cinq the day before it went off the road
Generally, I try to get the head/tail or other feature of the car in a top intersection of the rule of thirds But however, I have a diploma in photography so no point of me starting mentioning all that bumph as you'll get lost haha. Also, a low depth of field always does it for me, but again thats me. I like using my 50mm prime for those shots as it opens up to f/1.8.