King Juan Carlos, according to the 3000 Spaniards who participated in a television show called "The Spaniard of History" in May.
 
Miguel de Cervantes and Christopher Columbus (why, yes, in fact he was Italian, but apparently Spanish employees count), finished in 2nd and 3rd place, well behind King Juan Carlos. 
 
Those voting for the King cited his "decisive role in the transition and in the attempted military coup of Febuary 23, 1981, known as 23-F in Spain. Just a week ago Spain celebrated the 30th anniversary of the first Voting Day of the new democratic Spain - a key moment in the transition to democracy led by King Juan Carlos after the death of Franco.
 
You can read all about the program in this El Páis article. 
 
While the King was the overall winner, the results varied by age. The King won hands-down amongst Spaniards between the ages of 44 and 60, while voters under put Cervantes in first place, and the 31 to 43 year old group chose Christopher Columbus.

The overall top 10 also included Adolfo Suárez, elected president of the newly democratic government in that first election 30 years ago, Pablo Picasso and Santa Teresa de Ávila. Interestingly enough, Franco landed in 23rd place, not a finish he'd like, I suspect.

I've linked to the Spanish language wikipedia pages for each of the (linked) winners mentioned. Just a subtle suggestion to get reading in Spanish; you could always click over to the English version, but why would you want to?

So who do you think was the most important Spaniard of history, from an outsider's view?