Although I am with vodafone and it is fine, telefonica offer the best coverage. Being the main telephone provider they would do However their rates can be expensive – if you dont know how to get the better rates which means speaking Spanish.
However like in the uK they are all pretty much of a muchness and of the three main ones – vodafone, Telefonica movistar and amena, Amena seems to be the cheapest for most users, vodafone offer good rates but their points system means you get a new telephone eveyr few motnhs (if you want one) and movistar offer the best coverage where we are
Good luck