The problem

Which Spanish mobile company has the best network coverage?

My phone drops coverage intermittently while driving around.

How can I get better reception in Spain?

The answer

The actual infrastucture of the Spanish mobile network, which is mainly antennas, is owned and maintained by Movistar, Orange, Vodafone and MásMóvil. Their networks cover 99% of the population.  Most people live in cities so it is easy to provide a high level of coverage in urban areas.  Spain is a large country and there are large areas which are not highly populated. If you are driving around in the countryside there will inevitably be intermittent coverage in rural areas especially away from major roads.  If you look on internet for the normal maximum range of a 4G signal there is a lot of disagreement. From personal experience I know that is you have line of sight to a 4G antenna you can get 20 megas per second even at 16km away from the antenna. The signal of 5G has a much shorter reach than 4G so don't expect to get great 5G coverage in the countryside.

So which operator should I choose?

Although only 4 companies have their own network there are lots of operators in Spain.  The smaller companies use the networks of the bigger companies. Below is a table which shows you which operators use which networks. 

The answer to this question is that all networks have a similar level of coverage and the coverage should not be the reason to choose one company over another.  If you are driving around in rural areas the coverage will be intermittent. If you are using a non Spanish sim card then theoretically your phone should connect to the strongest signal but this might not necesarilly be the case.