There are many cars that are built so badly that it takes just a short time for them to become so rare that just seeing one on the road is special. What was the peak of that for you? In terms of age, it was the later Fiat 126p (by then renamed Maluch) for me. The production ended in 2000, but depreciation and low quality thinned the ranks so much that it was already having survivor status by the early 2010s. In terms of year, it was the Mk1 Ford Ka. The production ended in the mid-00s, but depreciation and rust have already made running examples relatively rare.