First visit: August 1977
Subsequent visits: April 2000, August 2002, September 2005, April 2006, March 2008, April 2009, November 2009, February 2010, May 2011, November 2011, February 2012, August 2012, November 2012.
Visited: Madrid, Barcelona, Sevilla, Valencia, Bilbao, Zaragoza, La Coruna, Burgos, Vigo, Pamplona, San Sebastian, Oviedo, Badajoz, Murcia, Santiago de Compostela, Santander, Lleida, Gijon, Logrono, Algeciras, Ceuta, Huelva, Puigcerda, Llivia, Llanes, Tudela.
A clear Mediterranean favourite, more approachable than Italy and more culturally diverse as well (imho). I have visited the extremes of the country in the south, northwest and northeast but only made it to Madrid in March 2008, though that wasn't overly impressive. The people are sometimes too hysterical, I feel, yet uncomplicated and friendly. It scores higher than even I would expect, as it gains good points for all aspects, including liberalism, infrastructure and way of life (but not food).
Highlight: Obviously Barcelona with its mix of Gaudi, the sea, wonderful nightlife and a Catalan twist - my favourite western European city.