The Amphitheatre at Pula, Croatia, is a fantastic sight, and is a real draw to the city. It is a lovely place, though, with a great atmosphere and some other really good sights which are worth seeing.
The temple of Augustus in the Forum Square is also very impressive, and the Arch of Sergius is an imposing presence just off the centre, too. There is also a Roman mosaic tucked away around the back of a shop of a side road. But nothing beats the view both inside and outside the amphitheatre. It really is amazing!