I've had two that stick in my mind.
One would be the first time I drove. Can't even remember the car, was a Corolla or something. I lived with my best friend through my high school years due to school expulsion and family issues that arose from that, so I never got my L-Plates while I was in high school due to lack of legal guardian. So the instant I turned 18 I hopped in a car with my friend's mum and just drove and drove for several hours.
Then, last year or the year before I was suffering from one of my usual bouts of depression I stormed out of the house, hopped in my Camry and just took a night drive around town...it was one of the greatest feelings ever, my mood just skyrocketed. However, it must have been something unique to that night because I have never been able to replicate that experience in any car since.
Another highlight (made me feel important) was tearing down the 90 mile straight on the Nullarbor Plain with my friends before getting pulled over by police. Though instead of arresting us all, the two cops insisted we give them a ride in our cars. Then, to top it all off, they drove ahead of us for a ways until we reached Caiguna at the end of the straight.
I've also had my share of bad experiences which I won't go into here.