Flights to Spain: Barcelona and the North-EastIf you're visiting Barcelona, fly to Barcelona airport. Sounds obvious, but Ryanair calls Girona and Reus (Tarragona on the map above) 'Girona Barcelona' and 'Reus Barcelona', despite the fact it will take you two hours to get to them from Barcelona.
Zaragoza is connected to both Madrid and Barcelona by high-speed train - good if you're doing a circuit, but otherwise stick to Barcelona flights.
Flights to Spain: Andalusia and the South
Malaga is the biggest airport in the south and is responsible for the city's tourism industry.
Malaga airport is well connected to other cities in the area. Only stay in Malaga if you have a particular reason to visit Malaga. There are far more interesting places in Andalusia to occupy your time. Read more about my list of the Best Cities in Spain.
Flights to Spain: Madrid and Central Spain
It's a no-brainer here: Madrid has the most flights from the most destinations at the lowest price.