Taking a flight to Spain is by far the quickest and most convenient method of travel, especially with the country's many international airports, and connections to the rest of Europe. A ferry is an option for those coming from the UK or from Morocco, otherwise bus - although long and draining - is the cheapest option.
Plane
Spain's International Airports include; Madrid, Barcelona, Bilbao, Santiago de Compostela, Valencia, Seville, Almeria, Granada, Málaga, Alicante, Ibiza, Palma de Mallorca, and Maó. A departure tax applies when flying out of Spain, but this is included in the price of the ticket at purchase.
Train
Travelling to Spain by train can be more expensive than by bus unless you are under 26 years old or have a rail pass.
Bus
Most major centers in Europe have regular bus services to Spain, including London, Lisbon, and Paris.