Depends on where you plan to go and with whom. For traveling around the country, a fresh small car is enough. Do not take old cars with small engines – the air conditioner does not work as you’ll want to. Especially in the mountains.
If you plan to travel a lot and far, take a bigger car. It would be ideal to rent a convertible for trips along the sea. It’s just an unforgettable experience. It makes sense to take an SUV only if you know for sure that you will need it. The roads are decent in all tourist places.