"What car do you regret not purchasing?"
None really come to mind. My regrets are for the cars left behind when I returned to the USA from military tours in Germany. My first tour ended in '89 I had a BMW 1802.
Not mine, but like this:
Second tour in the late 90s I had a VW delivery truck, Beer Wagon (and a variety of cars). It's diesel engine really labored going uphill, but it was a fun versatile ride, complete with a canopy for the bed.
Actual car:
I'd love to have either one of those again.