What Classic Cars Are Least/most Expensive To Maintain?

I would imagine it is easier to find parts for 1960s General Motors car than it'd be for a 1960s AMC because GM is still around. I don't know a lot about maintenance of cars, but any classic car I'd get would be worked on by my mechanic. What brands would be easiest to find parts for and hence least expensive to maintain? What brands most rare parts and expensive?