Here is a list of mobile phone companies in Spain.
Which is the best?
It is a very competetive market and it is easy to change companies so the prices and conditions of all compnaies are fairly similar.
A few years ago the Spanish government introduced "portabilidad" which stopped companies blocking you when you want to leave them.
What do you use?
I use movistar illimitado because it has multisim and unlimited data.
I use Simyo for an extra sim card which I use for an extra whatsapp number. With Simyo you can have a sim card almost for free and you only have to top it up with 1 euro every 3 months.
Here is a list of the Spanish mobile phone operators with links to their websites.
Pay as you go - Prepago
Use the internet - Navegar