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 suop for an extra sim card which I use for an extra whatsapp number. With suop 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.
It is also possible to use an esim if all you want is data airalo
Notes:
In theory by law you have to provide id when you purchase any SIM in Spain.
Most people in Spain use whatsapp to communicate with each other.
Essential Spanish:
Pay as you go - Prepago
Use the internet - Navegar
Click on the links to see the most popular activities that you can do in these places:
Note that the booking system uses a Spanish system and there is no involvement of any American company and no US tariffs.