I can't say that I have regretted getting rid of any of my cars, as I tend to keep them until they are completely stuffed by rust, or uneconomical to mechanically repair, or I just hated them after I bought them. Luckily, the current ones are still in good nick.
I would like to have my 186S HK, my HJ wagon, and my 5.0l VN Calais back again as new cars, but two were rust held together by paint, and the other was a mobile leaking oil drum.
The one that got away was a Ph2 GTHO for about $7k in 1986. Decided to buy it, test drove it, and then decided it would never live up to expectations as something to drive every day. Noisy, rattly, heavy, uncomfortable. And, with hindsight, I would have to spend big bucks to keep free from tin ants.
That one, the LH SLR and HQ Monaro are just like sands through the hour glass, etc .............