Magione and Nordschleife in that order give a good first impression and are a good way to learn cars in my opinion. Magione because it's very tight, has some good braking zones, turns that allow for some experimentation, one longer straight that leads into a somewhat aero-dependant turn that takes some courage to be fast, requires riding medium to high kerbs and is still short, good for quick testing. It's in a way like a smaller Vallelunga, only that it's a bit too flat. Nordschleife because it has everything, but it takes a long time to complete a lap in turn.
Imola, Barcelona, Vallelunga and Zandvoort are other good choices with good variety to put cars to the test.